The CCTV footage is in grainy black and white. But it is clear enough to see a man firing an AK-47 into the doorway of a crowded bar.

This is not downtown Mogadishu, but a local lunchtime news bulletin from suburban Philadelphia, the fifth largest city in the world’s richest country.

The story has not made it on to the television because of its shock value. It is presented almost as light relief because the gunman’s car rolled away as he fired.

In Philadelphia, violence has become a mundane fact of life, a grim reality that regularly spills over into its education system. Every day sees an average of 25 pupils and staff become the victims of a violent crime in one of the city’s public (state) schools.

Freire Charter is different. The high school represents an oasis of calm for the 750 teenagers from across Philadelphia who, every year, apply for just 160 places.

“There are no metal detectors here, like you see in other Philadelphia schools,” says 17-year-old pupil Mark Long. “That says a lot about the students. It shows the staff trust us. In here you feel safe. If you fight once, you are out.”

He is explaining the school’s strict “no second chances, no exceptions” policy of expulsion for any act of violence to a group of aspiring urban school leaders from England. They are in Philadelphia to learn about the high ambitions that “exemplary” charter schools like Freire can inculcate in students from the toughest neighbourhoods.

But as several pupils and teachers make clear to the English visitors during their tour of the city, often it is not academic standards that are the prime motivation behind applying for charter-school places, but basic physical safety.

That harsh truth is one of several preconceptions challenged on the trip for teachers being trained to head England’s most challenging schools. This is the sixth time that Future Leaders, a government-funded scheme, has sent trainees, and leaders from the schools they are placed in, to the US for inspiration.

But this academic year, the study tours - which also fanned out to charter schools in Chicago, Boston, New York and Washington, DC - had even greater pertinence.

Until September, American charter schools (publicly funded but independently run) were a piece of foreign exoticism for educationalists on this side of the Atlantic. They may have provided a rich source of ideas for leaders looking to improve state schools in Britain. But no one had to consider them as a whole with all their implications.

Now, with the opening of the first charter school-inspired free schools in England, that is changing. Late last year, as the English visitors discussed the pros and cons of a radical policy just starting in their own country, they were able to view the potential results several years down the line in Philadelphia.

And they did not always like what they saw. “There are schools (buildings) better than this in India,” confided one successful London head during the tour of DuBois Collegiate Academy, squeezed into two floors of a city-centre office block. “It is awful: all this fragmentation with schools in corner shops. If they want to improve education, why don’t they do it through the state system?”

Public schools ‘in disarray’

The answer, as far as those running the charter schools are concerned, is easy - Philadelphia’s public school system is “in complete disarray”. And with four of the city’s five-person School Reform Commission resigning last year, they have a point.

On top of this, Philadelphia’s public schools are facing budget cuts that are likely to make an already brutally unfair state funding system - its dependence on local property taxes means that the more deprived an area is, the less its schools are likely to receive - even tougher.

Charter schools in the city argue that their financial situation is worse still because the school district (equivalent to a local education authority) is allowed to retain a portion of the funding for every charter pupil.

But some charters benefit from private, philanthropic funding streams. And critics argue that they only make the job of public schools with the most deprived intakes harder by diverting per-pupil funding and enticing the most motivated students away.

Sue Thompson, head of academics at Freire, freely admits that charter schools can create problems elsewhere in the city. “These kids are choosing to be here and that does mean that other schools are losing these kids,” she says. “We have a significant advantage in that.”

For pupils from disadvantaged areas, unable to access the fee-paying or better-funded state alternatives available to the more prosperous, charters can represent the only way out of a violent, underfunded public school. But the more pupils who escape, the more unpalatable the default local option is likely to become for those who cannot.

Even if pupils are lucky enough to win a golden ticket in the charter school admissions lottery, they would be unwise to take their place for granted.

At DuBois Collegiate Academy, a Destiny’s Child track booms out in place of a school bell as pupils quickly and efficiently change lessons. The senior (15-18) high school is part of the teacher-founded Knowledge is Power Program (KIPP) and has an atmosphere that is both studious and upbeat. Inspirational messages about learning by everyone from Gandhi and Martin Luther King to Oprah Winfrey and rapper Mos Def stare down from the walls.

It is too early to tell whether KIPP’s classic “no excuses” charter model will produce results here. The academy is filling up a grade at a time and only opened in 2010. But one side of the school shocked some of the English guests during their visit. In its first academic year, DuBois lost a fifth of its pupils - some because families moved away, but others because “it wasn’t the right fit because of discipline or whatever”.

Staff explain that this did not usually happen through formal exclusion. Talks with parents took place before it got that far. Aaron Bass, the principal, admits that this “attrition” rate is too high. “That is not something we are proud of,” he says. “It hurts.” Experience with other KIPP schools shows the rate will fall over time, the visitors were told.

But KIPP’s overall attrition rate for Philadelphia is still 12 per cent and takes into account longer-established middle and elementary schools. When the visitors were informed that DuBois aims to lose no more than 10 per cent of its pupils over the course of a year, they were less than impressed.

This is the flip side of some charter school success and it is not confined to KIPP. At Freire, around one in 10 pupils leaves the school every year for a variety of reasons, including discipline or “a lack of alignment with the school’s mission”.

In theory, charter schools are open to anyone. But only children with parents ambitious enough to consider options beyond their neighbourhood school and take part in the admissions lottery have a chance of a place. And it seems that, even if they do get one, they must ensure they fit the charter school mould to keep it.

For the teachers from England, who work with some of the country’s most disadvantaged pupils and cannot ask them to leave if they “lack alignment”, the whole idea of an attrition rate is an alien concept.

A more familiar approach is exhibited by another charter-school operator on the tour. Mastery specialises in wholesale takeovers of existing, failing Philadelphia public schools, in a model akin to England’s academy programme. Becoming a Mastery school means new management and, potentially, a completely new staff, although existing teachers can apply. But the pupils and catchment areas stay the same.

“It is a big deal for us that we keep our students,” says Steven Kollar, principal of Mastery Lenfest, the chain’s first school. “We want to serve all of them.”

Last year, his school did see 6.3 per cent of pupils leave, but Mr Kollar says this is mainly explained by Philadelphia’s transient population.

The aim of lower attrition does not mean that the school has compromised the strict regime that makes other charters successful. With its clean, quiet corridors and attentive, on-task students, it appears to match the best of them. Mastery, a non-profit provider, has also managed to drastically reduce levels of violence in its former public schools.

“We feel, if our students are here, they are being academically successful,” Mr Kollar says. “There is no doubt that going to this school is more difficult. They are pushed harder and if they don’t show up they will get a telephone call that they won’t get in other places. They have Big Brother watching over them here.”

Nevertheless, while the school, with a 99 per cent African American intake, has results considerably better than the city and state averages for the same ethnicity, it is only on a par with African American pupils nationally and is significantly below the national average for all pupils.

Generalisation is unhelpful

Freire is another example of why it is unhelpful to generalise about charter schools. In many ways, the senior high fits the charter stereotype, with its cramped city-centre accommodation in a former YWCA building and an emphasis on academic achievement and university entry.

But then you discover that the school was inspired by Paulo Freire. A “progressive”, Brazilian, 20th-century educationalist, Freire attacked the idea of pupils as passive, empty vessels waiting to be filled with facts. Instead, he believed they should be active participants in an equal process where the teachers also learn and the learners also teach.

The idea is rammed home with a large sign in the school’s lobby carrying a message that might make those who emphasise the importance of basic subject knowledge, like education secretary Michael Gove, wince. “Imagination is more important than knowledge,” reads the Albert Einstein quotation. “Knowledge is limited. Imagination encircles the world.”

In other words, this is a school based around a philosophy representing almost the polar opposite to the stereotypical, back-to-basics charter approach with its didactic teaching and strict discipline. When Freire opened 12 years ago, it had no rules at all, let alone the strict codes of conduct that characterise most charter schools.

Since then, some regulations have been introduced at the request of pupils. But much of the original approach survives. Students refer to their teachers by their first names. And rather than the strict uniforms enforced by many charter schools, pupils at Freire can earn the right to ditch their school polo shirts if their test scores are high enough.

“You have a lot of freedom at Freire,” says pupil Mark Long. “There are no shackles here; it is shackle-free. This feels like my family, so I want to be here. I am as free as a bird, but I spend more time here than I do at home.”

The big differences within this tiny sample of schools make it impossible to draw firm conclusions about the merits of charter schools or the free schools they have inspired. But that was not why the teachers from England travelled to America.

They were there for ideas and found plenty of inspiration that they did not view as being reliant on a particular model of school governance.

“There is nothing I have seen in the charter schools that couldn’t be done in the state sector,” says Sheila Ball, a Future Leader working as assistant principal at the Ark Academy in Wembley, London.

Even the London head concerned about the “fragmentation” caused by charter schools was impressed by much of what they actually did. The head is now planning to import the explicit focus on university entrance they witnessed in Philadelphia to their own school.

There was also admiration for the emphasis on creating positive school cultures and the recognition of what was needed to raise the aspirations of pupils from poor, inner-city areas.

As Bass, principal at DuBois, memorably said, some of the pupils’ families have so little notion of what it might mean to progress to university that they might as well be “going to the moon”.

The English visitors were less impressed by the actual teaching they saw in Philadelphia’s charter schools and collectively rated it as no better than “satisfactory”.

But as Future Leaders chief executive Heath Monk points out: “If you can get kids that achieve and behave like that if the teaching is only satisfactory, imagine what you could do if it was outstanding.”

Future Leaders is recruiting on a first come, first served basis for its 2012 cohort. www.future-leaders.org.uk.