The gap in performance between independent and state schools is higher in the UK than anywhere else in the world, according to a report obtained by The Independent on Sunday.
The report, by one of the country's most respected researchers, also voices misgivings about an international study ranking the performance of English schools in maths, science and literacy as among the best in the world.
The findings will be particularly embarrassing to the Prime Minister, who makes a keynote education speech today, designed to switch attention from his U-turn on the European referendum and controversy over Iraq.
The international study, by the Organisation for Economic Co-operation and Development, has been seized on by ministers and regularly cited by them as evidence that the Government's school reforms are working.
However, a report by Professor Alan Smithers from Liverpool University's Centre for Education and Employment concludes the OECD findings are "flawed". It accuses ministers from Tony Blair downwards of "over-interpretation" of the figures.
The report argues the Government cannot take credit for British pupils' strong showing because New Labour policies had not come into effect at the time. The findings, based on tests taken by 15-year-olds and which were published in 2000, "can owe little to current policies" says the report. It adds that much of what success there was can be put down to independent schools.
Professor Smithers writes: "A factor in England's top ranking is the performance of pupils in independent schools.
"Across all countries (with the exception of Japan), independent school pupils did better than those in maintained schools, and those in England did best of all - with the gap from the maintained sector the widest."
Figures gave independent schools a points score of 614 on the reading test, compared with 515 for state school pupils, including those from grammar schools. The 99-point gap was higher than any other country in the survey.
"The independent schools, in fact, achieve the best scores in the world," adds Professor Smithers, whose research was commissioned by the Sutton Trust, a charity set up to help working-class youngsters secure places at top universities.
However, the professor argues that even if the contribution of the independent sector is ignored, the OECD results are still flawed. The UK came fourth in science, seventh in literacy and eighth in maths out of 33 countries in the study. Professor Smithers points out that the maths and science tests were restricted to just 30 minutes, compared with two hours for literacy, and that only some of the sample of youngsters sat them. In addition, the range of topics on which they were questioned was "very restricted".
"The maths testing was so limited one wonders if it can bear the weight that has been put on it," he says. "Furthermore, items were designed so as not to need particular calculation skills and the wrong answers could be awarded marks if an appropriate method had been adopted."
Despite this, he accepts that both performance in science and literacy levels in England are still above average, adding: "Taking the findings (from all surveys) at face value, it looks as if for England, the chief concern should be maths.
"Apart from the flawed test in 2000, the country consistently comes well down the ranking tables."
Sir Peter Lampl, chairman of the Sutton Trust, said: "The findings that our independent schools performed the best of any in the world indicate that not only those who can afford to pay should have access to independent schools."
Ministers will argue that they have taken action to improve standards by publishing an inquiry into the state of maths teaching in secondary schools and ordering a review of the examinations system.
TOP MARKS ALL ROUND - AT £2,000 A TERM
Fee-paying Manchester Grammar is one of the most academically successful schools in the UK. Last year a remarkable 99.5 per cent of its 16-year-olds were awarded five GCSEs at grades A*-C, and more than 50 of its sixth formers got places at Oxford or Cambridge. It even offers free Oxbridge tuition classes to students from neighbouring comprehensives.
James Jones, 17, from Stockport is in the lower sixth, studying maths, physics, chemistry and biology at A-level. Predicted to get straight-A grades, he hopes to read medicine at Oxford, Cambridge or London.
"I love it here. I wouldn't want to be anywhere else," he says. "The great thing is they teach you outside the syllabus. At some schools they can focus overmuch on the exams. At this place if you're enthusiastic and want to learn more, you can. The staff have real enthusiasm for their subjects. Everyone here is focused on wanting to do well."
Like other fee-paying schools, MGS runs dozens of out-of-hours teams, clubs and orchestras. James goes on regular mountaineering expeditions, plays tennis and belongs to the science society, which invites visiting speakers, most recently Heinz Wolff.
Dr Martin Stephen, high master at the £2,150-a-term school, said that for years the private sector has boosted national results in maths, science and English. But it was important not to "crow", he said, adding that many private schools are selective, and have no problem attracting good staff.
BOTTOM OF THE CLASS - BUT TRYING HARD
The Ramsgate School in Kent, a comprehensive where only 4 per cent of GCSE pupils get five A*-C grade passes, was at the bottom of last year's national performance tables. The same was true the previous year, and the year before.
Ramsgate is set to close later this year and reopen as one of the Government's city academies, sponsored by industry - in this case, Saga holidays - and run along the lines of a private school. But it is already under a new management, and things are improving.
Pamela Evans, 16, is doing seven GCSEs and hopes to go on to study childcare. Eventually she would like to start a business. Her schooling has not made it easy to fulfil her ambitions, she says. "Now we have teachers who say, 'You've got to do this', but in the past we haven't had teachers who pushed us.
"They didn't seem able to handle the naughty children. You wanted to do your work but you couldn't, because the others were disruptive."
Ramsgate is so focused on improving that there is little time for extra-curricular activities.
"I know there are some people around who get home tutors for extra English, but not at our school," Pamela says.
The school serves a seaside town with many asylum-seekers, so has pupils for whom English is not a first language. Bad publicity has made it even harder to change things for the better, says the new head, Keith Hargrave.
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