The government this week spelled out plans for the biggest upheaval to the schools system for decades, bringing an end to a 50-year consensus that there should be no new grammar schools. Here we outline the essential facts and reveal major outstanding questions, such as the issue of funding and how the government will decide which schools can select on the basis of ability.

What is the government proposing?

Ministers want to enable existing grammar schools to expand, as well as the creation of new wholly or partially selective schools, in response to “parental demand”, and to allow existing comprehensives to select their pupils on the basis of ability.

Why are ministers planning this?

The Green Paper talks of the need to increase the number of “good” school places. Prime minister Theresa May has said more selection would make Britain a “great meritocracy” and “shift the education system decisively to support ordinary working-class families”.

Why does she think more grammars will help?

The Green Paper bases much of its evidence on a Sutton Trust report from 2008. The study states that the “educational gain” from attending a grammar is “twice as high for pupils eligible for free school meals, when compared to the overall impact across all pupils”. It adds that 99 per cent of grammars are rated either “good” or better by Ofsted, and 80 per cent are judged “outstanding” compared with 20 per cent of comprehensives.

But aren’t grammar schools dominated by middle-class children?

The Green Paper itself acknowledges that as of January this year, just 2.5 per cent of pupils at grammar schools were eligible for free school meals (FSM). But the government has said that it wants to help more than just FSM pupils, and is looking to identify children in families that are “just getting by”, such as households earning around £20,000.

As part of its plans, the government has set out a “menu” of conditions that it hopes will mean more children from disadvantaged backgrounds will gain access to grammars. For example, it wants both new and existing grammar schools to provide a “proportion” of places to “low-income households”. But it does not say how they would do this.

What about the children who don’t pass the test and get into grammar schools?

The Green Paper admits there may be “an association with poorer educational consequences” for those who don’t go to a grammar. It even highlights a study showing that pupils in selective areas who fail to get into the local grammar perform worse by one GCSE grade point than their peers in comprehensives in non-selective areas. There is also a substantial body of evidence suggesting that grammar schools do little to improve social mobility.

How do ministers think new grammars will counteract this effect?

As part of the menu of conditions for expanding selection, the Green Paper proposes that new and existing grammars could establish a non-selective secondary school in the same area. It states that grammars could set up a “feeder primary school” in areas with high numbers of low-income families. It also says grammars could partner with a comprehensive in a multi-academy trust or sponsor a non-selective school.

There is a substantial body of evidence suggesting that grammar schools do little to improve social mobility

Who is supporting the plans?

A large section of the Conservative Party has spent decades campaigning for a return to a national grammar schools system. Existing grammars and selective local authorities, such as Kent and Buckinghamshire, are very supportive, as they have been keen to expand for a number of years.

What about education secretary Justine Greening?

It is not without irony that the first education secretary to have been schooled in a comprehensive will also be the first to expand selection. Ms Greening has consistently stated that she is “open-minded” about selection, and that it is “worth having the debate” on the expansion of grammar schools. But sources within Westminster have suggested that she is “not overly enthusiastic” about the policy.

Who is against?

Opposition to the proposals is widespread. The Fair Education Alliance - with members including Teach First, the Ark academy chain, the CBI business group, the University of Oxford, both headteachers’ unions, and university admissions body Ucas - has set up a petition resisting the plans. It was followed this week by research from the influential thinktank the Institute for Fiscal Studies, which warned that an expansion of selective education would “increase inequality”.

What about public opinion?

The Green Paper states that grammars are “popular with parents”, and a YouGov poll last month showed that 59 per cent of the British public supported plans to lift the ban on creating new grammars.

And teachers?

A TES snap poll of teachers this week found that 72 per cent were opposed to the plans, while 56 per cent said they would not be prepared to teach in a grammar school. In a survey of teachers and heads by the NAHT headteachers’ union, the Association of School and College Leaders and Teach First, four out of five were against the proposals.

Does that mean it is unlikely existing comprehensives will apply to select?

No. According to NAHT general secretary Russell Hobby, while the majority of heads would be reluctant to select, there would also be a “fear of being left behind”. He added: “It will take just one school in one area to opt for it, and other schools will feel they will have to.” One source told TES that as yet Number 10 “had no answers” as to how it would decide which schools would be able to select.

How much will all this cost?

The only concrete figure to have been committed to so far is £50 million a year to help existing grammars to expand. It is expected that the creation of new selective schools will come from the existing free schools budget. Last year, the government committed to opening another 500 free schools by 2020, which the Liberal Democrats claimed would cost £4 billion.

But the requirement for grammars to create new non-selective schools is likely to mean even more spending. Malcolm Trobe, interim general secretary of the Association of School and College Leaders, said any major restructuring “will place a financial strain on the system”. And he warned: “This is an unnecessary distraction to key issues that need urgent attention - most importantly, the supply of teachers.”

What happens next?

The consultation runs until 12 December. According to the Green Paper, the government will respond - possibly in the form of a White Paper - by March 2017.

Will the grammar school proposals make it through Parliament?

It depends on the government’s reaction to the consultation. TES understands that Conservative modernisers are working behind the scenes to oppose the plans. One source said they were “keeping their powder dry” to wait and see how the government responded.

According to estimates, about 40 Tory MPs are willing to rebel, while the Scottish National Party has said it would be prepared to abandon its stance of not voting on English laws to “fight tooth and nail” if it felt the plans would affect Scotland.

But the government faces its greatest challenge in the House of Lords. As the grammar policy was not in the Conservative manifesto, the Lords are not bound by convention to support the proposals. That raises the prospect of a snap election if Ms May wants to push it through.

@RichardVaughan1