Private prep school pupils who go to grammars arrive at their new schools with poorer key stage 2 results than their classmates from state primaries, new research shows.
Professor David Jesson of York University says his research shows that parents are buying their children advantage by using prep schools to coach pupils through state grammar schools' 11-plus entrance exams.
The influential academic, who is also an associate director of the Specialist Schools and Academies Trust, says private schools can take advantage of greater freedoms than state schools to concentrate on coaching for the 11-plus. They do not have the same pressure as state schools to focus on KS2 Sats.
Professor Jesson's research shows that in 134 of England's 164 remaining grammars, pupils who has been to state primariess had done better in national tests for 11-year-olds than those educated in the private sector.
"For the great majority of grammar schools, the key stage 2 points scores of pupils from private schools was substantially lower than those from state primaries," Professor Jesson writes in Research Intelligence journal.
"It is surely very surprising to find that, with such competition for entry, able pupils from the state sector appear to be discriminated against in favour of less able pupils from the private sector."
His figures are published as competition for grammar school places hots up, particularly in London where higher-than-average proportions of children attend private schools.
The credit crunch and rising school fees mean that many of the moneyed middle classes can no longer afford the private secondary places they once took for granted.
Nationally, around 7 per cent of pupils are educated privately. But Professor Jesson's figures show that 15 per cent of the intake in 164 grammars attended private schools.
In 50 of the grammars, the proportion was even higher, between 15-20 per cent, and another 20 grammars took more than a quarter of their pupils from the independent sector.
Professor Jesson's paper says these independent feeder schools have an inbuilt advantage. "Private schools are not required to follow the national curriculum and are therefore free to coach for grammar school entry tests - one reason why parents choose these schools in the first place," he writes.
"The contrast with pupils in state primary schools could hardly be sharper. These pupils have, by law, to follow the national curriculum, and any coaching would have to take place outside of school and at substantial additional expense."
This, he claims, is why grammar schools on average have just 2 per cent of pupils eligible for free school meals compared to a national average of 13 per cent.
However, another explanation for prep school pupils' lower KS2 results may simply be that their schools see the tests as less of a priority - if they do them at all. Although prep schools have historically outperformed state schools in the tests by a wide margin, increasing numbers have abandoned them altogether.
The Independent Incorporated Association of Preparatory Schools revealed two years ago that only around a third of its members bothered with the exams.