The recent row over back-door selection shows that some schools are willing to break the rules if it will land them the most able pupils.
But both main opposition parties are backing a policy which could actually lead to schools competing to teach the most disadvantaged children. The "pupil premium", supported by the Conservatives and Liberal Democrats, would attach a nationally set amount of extra funding to young people from deprived backgrounds.
A change of government at the next election could lead to an increase in budget and staffing of about a third for schools such as Stretford High, a secondary modern in Trafford, Manchester, where large percentages of pupils are eligible for free school meals.
Derek Davies, its headteacher, is excited by the possibility. "This would make a huge difference," he said. "You could almost give each pupil their own personal mentor."
For the time being the operative word must remain "could".
Only the Liberal Democrats have calculated the figures, and even they would probably admit there is little chance of them controlling the schools system in the near future.
Nevertheless, the sums they are proposing are considerable. The plan would see school funding for England's poorest pupils soar by more than 60 per cent to the independent sector levels enjoyed by the better-off. And while the Tories may not yet be putting their money where their mouth is, they have agreed in principle to the policy.
It would not only mean schools receiving several thousand pounds extra for every disadvantaged pupil, giving them greater resources to improve their education, but would also be a huge incentive for them to admit them in the first place.
Evidence of how seriously the idea is being taken comes in a series of articles being published by the Institute of Economic Affairs' (IEA) journal in June, in which both politicians and policy think tanks begin to get to grips with the detail.
Proponents of the scheme have faced a tricky choice since they first began investigating it more than two years ago. They would either have to find large amounts of extra money, or face the prospect of redistributing existing school funding with the inevitable consequence of the electorally important middle classes losing out.
The Liberal Democrats have now opted for the former. Nick Clegg, their new leader, told The TES that the party would inject an extra pound;2.5 billion into schools, funded by changes to the tax credits system and non-schools public spending.
This would allow an extra pound;3,400 per head boost for the 14.5 per cent of pupils eligible for free school meals, on top of the current overall average funding per pupil of pound;5,430. The vast funding gap they now have with independent pupils would be closed immediately.
The Conservatives are at a much earlier stage, but have made a public commitment to use extra funding to skew the schools "market" in favour of disadvantaged pupils.
Perhaps surprisingly for a party supposedly wedded to helping the poorest in society, Labour has not adopted the idea, despite support from former cabinet minister Alan Milburn, and Julian le Grand, one-time adviser to Tony Blair on public affairs.
The Government does try to help disadvantaged pupils through the current school funding sys- tem. Deprivation money is divided between local authorities according to their overall levels of disadvantage, but does not always reach its intended destination.
As Paul Marshall from Centre Forum, a think tank, points out in his IEA article, the Government's own analysis has found that at least one authority is known to have divided its deprivation funding equally among schools, regardless of their intakes.
Potential problems with the pupil premium have also been raised.
Professor Alison Wolfe, of Kings College in London, writes that, post-16, what matters is persuading disadvantaged young people to stay in full-time education. A pupil premium would do little to improve this participation rate because the extra money is for the institution rather than the individual.
Research has shown that family background makes little difference to the chievement of 16- to 18-year-olds when prior attainment is taken into account.
Working out which pupils would qualify for the extra funding is a potential difficulty. Some have said the free school meals measure, suggested by the Tories and Lib-Dems, is too crude and "all or nothing".
Sam Freeman, of think tank Policy Exchange, fears the premium could create instability, with dramatic year-to-year fluctuations in school funding.
However, providing the social composition of a local area and a school's admission policy both remain relatively constant, there is little reason to expect the scheme to prove so volatile.
But if you are going to pump vast amounts of extra money into schools to help disadvantaged children, then you have to work out what you are going to spend it on.
Mr Marshall suggests smaller class sizes, extended hours, catch-up and booster numeracy and literacy classes, and bonuses to draw in the best teachers.
But the benefits of the pupil premium could be wider still: it offers the chance of achieving a more socially balanced schools system.
Under the plan suggested by the Liberal Democrats, Stretford High's current pound;3 million budget would increase by a massive pound;840,888.
Around 36 per cent of its 687 pupils are eligible for free school meals. With the annual cost of a learning mentor around pound;20,000, the funding would allow head Derek Davies to almost double the number of non-teacher adults he has working in school to help improve pupil achievement.
That would give him the resources to provide sessions that would also draw in the families of disadvantaged pupils. "It is about working with parents and the whole community to raise aspirations, rather than just focusing on exams," he said.
But he added that, to enable him to employ more staff, he would have to be sure the extra funding was permanent.