Hundreds of schools face being wrongly marked down by inspectors using a new system of evaluation, heads have warned.
Inspectors have been accused of using data, known as contextual value-added (CVA), in a "woodenheaded way", instead of using a range of information and observation to make their judgements.
Heads allege they arrive having pre-judged their schools on the basis of this data. They then find it hard to shift their view if the evidence counters the bare statistics.
Ofsted has been forced to warn inspection teams that these statistics should not be used to determine verdicts. But it said the system, the most sophisticated measure yet used to assess the quality of schools' exams results, offers them powerful insights into performance.
The new system was launched in September, with primaries and secondaries evaluating their own performance and inspectors then carrying out short visits to check these findings.
Since January, CVA, which seeks to identify how much progress pupils make taking into account their characteristics, such as free meals, gender and ethnicity, has been used as a major criterion in reaching Ofsted grades.
Schools say this method penalises those which have either many high or many low-achieving pupils.
Ofsted's is not the only measure of CVA. Another version, developed by the Fischer Family Trust charity, is giving heads alternative results.
The trust has said that for one in five secondaries the judgement of whether a school is significantly better or significantly worse than the average changes according to which measure is used.
Heads claim many inspectors view the Ofsted measure as the final word on a school's quality. If the Fischer Family Trust data is taken into account, many ratings would change, they say. And it is not clear to them why Ofsted's model is better.
John Dunford, general secretary of the Association of School and College Leaders, said: "School performance is being misjudged by Ofsted. Up to one in five schools may be being misjudged." Dr Dunford has written to the Government to complain that heads have been told by inspectors that a school's CVA score prevented them from giving it a good rating.
Professor David Jesson, of York university, said too many inspectors were taking a wooden-headed approach to the use of CVA, building up a picture of a school based on the data, which was not changed by an inspection.
The problems are not confined to secondaries, according to Mick Brookes, general secretary of the National Association of Head Teachers. He said CVA was being used as a "bat to hit schools with".
John Chowcat, general secretary of the inspectors' union Aspect, said: "I do not believe inspectors are shutting their eyes when they do their visits. They are listening to what the heads have to say. CVA is a major influence, but not the only influence, on their decisions."
A Department for Education and Skills spokesman said CVA was the best way of measuring school performance.
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