Back in 1989, American student Wendy Kopp came up with an idea. High-flying graduates, she proposed, should be recruited to teach in low-performing schools in the most impoverished communities.
There was no need to wait for them to complete the traditional year-long teacher training programme, she insisted. Rather, the recruits should be sent on a short, intensive "boot camp" to learn the basics, before being parachuted straight into schools with a teacher shortage. The graduates could then choose to stay in the classroom or go on to work in other sectors.
After reading her thesis on this idea, Kopp's adviser at Princeton University was initially less than impressed with her plan for a "domestic Peace Corps" to tackle educational inequality, telling her she was "quite evidently deranged". But the dissertation went on to receive an A grade. Kopp sent her proposal to the heads of several major corporations; the day after her graduation, oil giant Mobil (now ExxonMobil) approved funding of $26,000 (pound;16,000) for the ambitious programme.
In its first year the scheme, called Teach For America, placed 500 eager graduates in tough schools. In 2012, 22 years on, the programme received more than 48,000 applications, resulting in 5,800 teachers being posted to schools across 46 regions in the US, from Alabama to Washington.
Today, Teach For America has an annual budget of $350 million. The organisation boasts that it has helped more than 32,000 participants to work with more than three million children nationwide over the past two decades.
But while this rapid ascent from student start-up to political powerhouse has been nothing short of remarkable, the genesis of Teach For America was only the start of the story.
In 2002, social entrepreneur Brett Wigdortz founded Teach First, a similar UK scheme, which has grown from an initial cohort of 180 to become the biggest graduate recruiter in the country. By 2015, it is expected to be taking on more than 2,000 graduates each year.
With interest in the two programmes growing around the world, Wigdortz and Kopp joined forces in 2007 to create Teach For All, a network to promote and support the spread of their shared ethos to countries around the world.
Today, Teach For All has more than 30 international partner organisations spanning six continents. From Austria to Australia, Bulgaria to Brazil, it has become one of the most successful and influential movements in global education.
This is not to say that support for Teach For All is universal. Concerns have been expressed on both sides of the Atlantic about the ethics of "experimenting" on children from the poorest communities with inexperienced teachers.
The zeal with which the Teach For All movement promotes its achievements, and cultivates support - both financial and political - from politicians and businesses, has led some critics to portray it as an educational "cult" wielding enormous power over policymakers.
Arguably most damning is the accusation that the programme is being used to staff struggling schools on the cheap, with eager graduates more interested in boosting their CVs than improving the lives of vulnerable children. Some even claim that the schemes bring little tangible benefit when the significant amounts of public and private funding being thrown into them are considered.
But the Teach For All movement is quick to counter the naysayers, pointing to numerous pieces of research proving, it says, that it delivers both value for money and real results for students. Indeed, it argues that some of its recruits even outperform more experienced colleagues who have come through the traditional teacher training route.
England's schools inspectorate Ofsted judged Teach First's training provision to be outstanding in every category, and praised its encouragement of participants' "relentless focus on the learning and progress of their students".
A key piece of 2004 research on Teach For America found that average test scores in maths classes taught by participants were higher than those of other teachers.
More recently, US research published by Mathematica Policy Research last September concluded that middle and high school teachers who had come through the programme were "as effective as, and in some cases more effective than, other maths teachers in the same schools".
And, in the UK, research by Dr Rebecca Allen of the University of London's Institute of Education has found that Teach First increases GCSE grades across participating schools.
Wigdortz credits the programme's success to the passion and panache of its "pioneers", who first set foot in the classroom more than a decade ago. "Every staff member was in [their] twenties - we had not a lot of experience," he says. "Now the first cohort is all in these amazing leadership roles. I think it's because they had this amazing get up and go."
Taking a risk
John Rendel was one of the first Teach First recruits, and spent two years in the classroom at Sacred Heart Roman Catholic School in Camberwell, South London.
The introduction to the profession was tough, he admits. "There's no harder job than teaching," he says. "For a new young teacher in the UK in a difficult state school, behaviour management is something you're thinking about all the time. It's about just keeping control.
"But if you teach well, the rest follows. It's easy to concentrate on behaviour instead of getting the students interested and motivated."
The Teach First pedagogical philosophy, he explains, is all about the "internal locus of control".
"There's no point in blaming the kids or parents. You're the adult in the room," he says. "While that's hugely stressful, as it places huge responsibility on you, it also means that you recognise that in this room, if this group of kids is going to learn, it's you that's the major driver of that.
"When I signed up, I didn't have a clue Teach First was going to be a success. [At the start] it was a slightly risk-taking, entrepreneurial cohort of people. We felt that if it was going to work, it was going to work on the back of us being successful.
"It was all about `esprit de corps', `learning to lead'," Rendel laughs. "There was some great lingo, partly due to Brett [Wigdortz]'s American background, I think."
Although he is no longer a classroom teacher, Rendel's Teach First experiences kindled a lasting passion for education. After Teach First, he went on to found the charity Promoting Equality in African Schools, a chain of 22 low-cost private secondary schools that educate 8,000 students in Uganda and Zambia.
And the influence of Wigdortz and Teach First on Rendel's outlook has been profound: "I don't know whether Brett would call himself a mentor, but I certainly think of him as one, in terms of an Obi-Wan Kenobi-type figure. He's done absolutely extraordinary things with Teach First.
"In the education world in the UK, Teach First plays a very important role. That's only going to grow. The power of Teach First is as much about what Teach Firsters will go on to do as what they're doing in the classroom."
Teach For Bulgaria is one of the newest Teach For All members. Founded in 2011 by entrepreneur Evgenia Peeva, the organisation aims to tackle some of the fundamental problems at the heart of Bulgaria's education system.
In the most recent Programme for International Student Assessment (Pisa) rankings, released last month, Bulgaria was ranked 47th for maths and 51st for reading, lagging behind neighbouring countries such as Turkey and Romania.
"It really does not make sense for a significant [proportion] of kids to be behind just because of their background," Peeva says. "But in Bulgaria, this is seen very clearly: 40 per cent of kids do not have the minimum required level of functional literacy needed to be successful in their lives and their jobs.
"The gap between the top performers and the bottom performers has been increasing over the past 10 years, so we have a difficult challenge."
Teach For Bulgaria also aims to tackle another problem: an ageing workforce. "The majority of teachers are 45 years or older," Peeva says. "There are very few young people who want to come into teaching. The teaching profession is not prestigious at all: most young people who go into it do so because they don't have other options or don't know what to do."
Unusually, the Bulgarian programme is open to teachers who are already qualified, as well as graduates. Both groups are attracted by rigorous initial and ongoing training programmes.
And the first cohorts that have come through the scheme have embraced the Teach For All philosophy, Peeva insists. "What's great is the attitude of our teachers. They realise they are far from perfect, they have a lot to learn," she says. "What they need to be doing is not only working hard and being really thoughtful about how they help their students progress, but also learning from their colleagues and being open to all the experience they can get from them."
Whereas most of the Teach For All programmes are targeted at working-class students in deprived urban areas, Teach For China focuses on driving up attainment in rural schools, explains Phil McComish, Teach For All's vice-president for network impact. "People migrate to cities and they get good education in cities, which are developing economically very quickly, but the rural areas get left behind," he says.
And while the US and UK programmes may be the most established, the newest Teach For All schemes, even those in developing countries, sometimes have a few things to teach the old guard.
"A couple of years ago, the Teach First team from the UK went to India and saw a lot of the work they do there with local communities, building coalitions within communities and listening to what community leaders want," McComish says. "It's similar to the way Teach First is now building alliances alongside other organisations in the UK.
"The structure of Teach For All means we have people doing unconventional business routes. It's very unusual to get Peruvians flying to India, or Pakistanis going to Bulgaria. We find the knowledge flow doesn't necessarily follow socio-economic patterns; the organisations with the most innovations are sometimes in poorer countries.
"When people talk about development work, it's about rich countries giving to poor countries. Actually, sometimes in our network we have poor countries helping rich countries. It's the power of education in that way: if you have strong ideas and strong school systems and innovations, there's so much about education that's universal."
But, for Wigdortz, one of the most significant consequences of Teach For All has been that the programme's prestige has helped to "detoxify teaching". The selection process is rigorous: in Teach For America, for instance, only one in eight applicants is selected. "I think that it's the most difficult graduate job out there," he says. "This isn't a job for the faint of heart. You need to have real leadership, lots of resilience. We look for respect, humility, lots of skills. I think many people would struggle with it."
The Teach For All movement does, however, have its detractors. While numerous pieces of research have demonstrated the positive effects of individual Teach For All schemes, critics argue that the overall impact has been minimal and significantly overhyped, particularly considering the huge financial and political backing the projects have received.
At the Free Minds, Free People education conference in Chicago, US, last summer, a meeting titled "Organizing Resistance to Teach For America and its Role in Privatization" was held. While it may sound like a fringe event for left-wing activists, it was actually set up by several former Teach For America participants whose experiences had turned them against the movement.
Negative critiques of the Teach For All ethos floating around the education blogosphere refer to "indoctrination and brainwashing", the "cult of saviours syndrome" and "evangelical zealots". In particular, the assumption that the Teach For All movement is a more effective means of eradicating inequality than the conventional teaching route has provoked hostility.
Last year, a UK fundraising campaign by Teach First calling on supporters to "Give pound;3 to sponsor the lesson that could start to change everything" came in for stinging criticism from educationalists on social media, who argued that it was patronising towards teachers outside the scheme.
As blogging headteacher Tom Sherrington put it at the time: "There is no evidence that [Teach First] does anything to tackle poverty any more than all the other [non-Teach First] teachers; I think it is insulting to mainstream teachers working in the same environment."
Some opponents of the Teach For All movement have an even more serious objection: that the scheme brings no tangible benefits.
Julian Vasquez Heilig, associate professor of educational policy and planning at the University of Texas at Austin, is one of Teach For America's fiercest critics, and has written several blogs and research reports questioning its effectiveness.
"The approach is that we can take anyone with a college degree and just put them into the classroom almost immediately," he says. "What Teach For America actually is is a temp agency. It's just a stopover; [participants] do it as a resume builder. It's a cheap route to staffing schools."
And while high-flying young graduates may offer an improvement on the weakest teaching that exists in some schools, Heilig is concerned at what this means for the most vulnerable students in the education system.
"The only choice in these schools is between a bad teacher and a worse teacher," he says. "Is it a false choice? The poor haven't been given the option of having a good teacher. What the poor kids get is ill-prepared teachers."
Heilig also questions much of the academic evidence produced to demonstrate Teach For America's effectiveness. "The research they [commission] paints their programme in a very favourable light," he says. "There is very little peer-reviewed academic research."
He cites the example of the recent Mathematica research into Teach For America. "They use methods that look complex, but when you take them apart they are problematic," he claims.
The research found that, among secondary maths teachers, Teach For America participants recorded an improvement of 0.06 of a standard deviation in student attainment, compared with their conventionally trained peers. This impact is "minuscule", Heilig argues, especially when compared with other educational initiatives.
So are these criticisms of Teach For All programmes justified? When asked this question, Wigdortz momentarily bristles. "I'm not aware of any other teacher training programmes that have had the impact Teach First has had [in the UK]," he replies. "It's down to headteachers to hire our teachers; they're not being forced on any school. If a headteacher thinks our teachers aren't doing a good job or making an impact, then they should not hire our teachers."
So what lessons does Teach For All have for the wider educational world? "We have a very strong vision," Wigdortz says. "It is that young people from deprived communities should be able to achieve at the same educational level as young people from wealthier communities.
"I'd love to see all teacher training routes have this strong ethos about what it means to be a teacher."
Tough Young Teachers, a documentary series following six Teach First recruits in their first year of teaching, begins on 9 January on BBC Three.