The most able children are only half as likely to achieve top grades at A level in state schools as they are in the fee-paying sector, a government adviser told head teachers.

Pupils in private schools who were among the country’s brightest 5 per cent at age 11 were virtually certain to get three A grades in their A levels at 18, putting them in contention for places at Oxford and Cambridge.

But only a third of the most able 5 per cent went on to achieve the same results in state schools.

The research was presented by David Jesson, an education evaluator based at York University, who said that the state system was suffering a “severe talent drain”.

He told heads at the annual conference in Birmingham of the Specialist Schools and Academies Trust (SSAT), which represents 2,422 state secondary schools, that Britain’s future economic success depended on identifying and nurturing such children.

The findings present a huge challenge to Ruth Kelly, the Education Secretary, as she fights to win Labour support for reform of secondary schools.

Many backbench MPs are suspicious that plans to turn comprehensives into independent trust schools will lead some to introduce “backdoor selection” of bright children.

Professor Jesson’s findings came from research that tracked the progress of the brightest 5 per cent of pupils between 1999 and 2004, based on scores in national curriculum tests of English, mathematics and science at age 11 in primary schools. He was given access to the data by the Department for Education and Skills (DfES).

Professor Jesson said that it was a myth that the brightest children attended private schools.

In fact, of the 37,500 children in the top 5 per cent, 30,000 went on to state secondaries and 7,500 were educated privately. By the age 16, all 7,500 in fee-paying schools had achieved at least five GCSE grades A* or A. But only 20,000 of the original cohort in state schools reached this standard.

The professor said that 13,000 students in state schools achieved three A grades at A level. In independent schools, the number was 7,600.

“At age 11, 7 per cent of all pupils are in independent schools. By age 16, 25 per cent of those achieving five A* or A grades are in independent schools. At 18, 33 per cent of those with three As at A level are in independent schools, and 44 per cent of Oxbridge entrants,” Professor Jesson said.

“There is the evidence not merely of a state-independent school divide, but of a state-independent divide on pupils who are similar. This is evidence of a severe talent drain.”

Oxbridge admitted 3,500 candidates from the state sector in 2004 and 2,600 from independent schools. Bright children in independent schools therefore had a 1-in-3 chance of getting into Oxbridge compared with a less than 1-in-8 chance for students in the state sector.

Professor Jesson, who is associate director of the SSAT, urged heads to demand the test performances of individual 11-year-olds from their local authorities.

They should use the data to set targets for achieving high numbers of GCSEs at A* and A, rather than the usual A* to C, and “seek the support and encouragement of parents” to help pupils to reach their potential.

In another study, researchers found that the £1 billion programme to raise standards in inner-city secondary schools has had little impact on academic results.

There was “no evidence” of improvements in GCSE results in schools that were part of the Excellence in Cities (EIC) scheme, started by the Prime Minister in 1999, researchers said. Attendance levels improved marginally, according to a national evaluation published by the Department for Education and Skills, and there was some improvement in mathematics in younger students in the most disadvantaged schools.

However, there was no evidence to show that EIC had an impact on attainment in English or science at the end of Key Stage 3 or Key Stage 4.

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