Ofsted is judging schools on the academic ability of students, rather than on the quality of the education offered, analysis of the inspectorate’s reports suggests.

There is a striking correlation between schools’ Ofsted grades and the prior attainment of their pupils, according to research into more than 2,600 judgements of secondary schools. Trevor Burton, the headteacher who carried out the analysis, described the picture it reveals as a “scandal” and has joined those calling for the inspectorate to be reformed.

The link between prior attainment and Ofsted grades is particularly stark in England’s grammar schools, which, by definition, take only high-ability students. The analysis shows that 76 per cent of the selective secondaries were deemed to be outstanding in inspections up to January 2014. But only 13 per cent of secondary moderns (schools in the same counties as grammar schools) and 19 per cent of comprehensives achieved the top rating.

Conversely, just 1 per cent of grammars were judged to require improvement or to be inadequate, compared with 28 per cent of secondary moderns and 30 per cent of comprehensives.

“Do you really believe that all the best leaders are in grammar schools? I don’t think so,” said Mr Burton, headteacher of Millthorpe School in York. “Whether it is the inspection teams Ofsted have got on the ground, or the [inspection] framework, there are some serious flaws in there and I want them fixed. I am in favour of inspection but it has got to be right.”

Mr Burton also conducted a detailed comparison of prior pupil attainment - as measured by primary national test scores - and Ofsted’s latest verdicts for every secondary in England where the data was available.

This shows the same marked trend as the grammar school results: the more able a secondary school’s pupils are on entry, the better their Ofsted rating is likely to be. More than 80 per cent of schools in the top two bands for prior pupil attainment were judged to be outstanding, compared with less than 20 per cent in the bottom two bands. Ofsted has not disputed the figures but insists that its inspectors pay “close attention” to prior pupil attainment and take a broad view of schools.

The inspectorate has faced sustained criticism over the quality and consistency of its reports on schools. Sam Freedman, head of research at Teach First and a former policy adviser at the Department for Education, has called for the abolition of judgements on teaching quality, saying that the chances of them being reliable are “infinitesimal”. Professor Robert Coe of Durham University has expressed similar concerns and called on the inspectorate to provide evidence that its methods are valid.

Ofsted is aware of the issues surrounding grading individual lessons and is running a pilot scheme to just observe and not grade lessons.

Responding to the research, Ian Widdows, who belongs to a newly formed association of secondary moderns, said Ofsted demonstrated “extreme bias” against such schools. And Chris Wallis, headteacher of Giles Academy, a secondary modern in Boston, Lincolnshire, said: “It is clear evidence, beyond reasonable doubt, of partiality and prejudice.”

Ofsted’s chief inspector, Sir Michael Wilshaw, said in December that grammar schools were “stuffed full of middle-class kids”. “I don’t think they work,” he argued, adding that they “do well with 10 per cent of the school population, but everyone else does really badly”.

Mr Burton’s research finds that of the 157 grammar schools that received Ofsted grades by January, just one was deemed to require improvement and one to be inadequate. As well as better exam results, Mr Burton said progress measures favoured selective schools because they were based on primary test scores that did not demonstrate how far ahead grammar pupils were on entry.

He added that grammar schools benefited from the affluent backgrounds of their pupils, whose behaviour was likely to be better. “As a headteacher I want to be judged on the difference I make,” Mr Burton said. “It seems to me that headteachers are being judged on the type of children that come into their schools.”

An Ofsted spokesperson said: “Ofsted inspects a broad range of criteria, which includes pupil progress, quality of teaching and behaviour. Final judgements take these numerous factors into account. Inspectors pay close attention to prior attainment starting points and look at how pupils make progress.”