More than 5,300 schools with below-average test results will be failed by Ofsted from next year, unless they can show they are "closing the gap", confidential documents reveal.
The TES has seen an inspectors' guide to the watchdog's new framework - being piloted this term - which shows it will be even tougher on schools' raw results than previously, regardless of pupil background.
And even more schools could be hit if their results for a particular category of pupils - such as boys or children in care - are below average for that group.
Under Ofsted's new regime, being introduced in January, the schools would be placed in special measures or given a notice to improve, except where they are "improving steadily and therefore closing the gap with the national average for all pupils".
Heads' leaders say the watchdog's plan will unfairly penalise schools with pupils from tougher backgrounds.
Many schools could be failed by Ofsted even if they have met Government floor targets, which often results in job losses and recruitment problems.
The pressure on schools with below-average scores appears to go beyond those seen as "coasting" and takes in any that are not improving faster than the national rate.
William Parker School in Daventry, Northants, meets the GCSE floor target but will fail its next Ofsted inspection in 2012 unless it significantly improves its results this summer, under the new guidelines.
Headteacher Jason Brook is responding by reluctantly introducing BTECs to push up scores. "Judging on national averages is crass and blunt," he said. "They need to take account of schools' individual circumstances. We were delivering the curriculum that we knew was right. But I can't afford to do that any longer. I have got to join the game."
The new framework could also penalise secondaries that slip back on the main GCSE measure because of their desire to improve on the English Baccalaureate, which will not count in Ofsted inspections.
Brian Lightman, general secretary of the Association of School and College Leaders, said: "What we don't want is more perverse incentives to play the league tables. Inspections should be looking at the overall quality of education."
The existing inspection framework was also controversial for an increased emphasis on raw results. But its targeting of "low" and "significantly below average" scores is now being ratcheted up to "below average".
Contextual value added, which adjusts exam results with background factors such as poverty, had played a key role in Ofsted judgments.
But, as The TES revealed last week, ministers are abolishing the measure because they say it "entrenches low aspirations".
Russell Hobby, general secretary of heads' union the NAHT, said he had been assured the new inspections would take more account of pupil progress.
"If a school achieves great progress with pupils that come in with low attainment then they are doing a remarkable job, even if the final results are below the national average. That is not failing."
Of the state primaries that had Sats results last year, 3,883 - 39 per cent - were below the national average on the percentage of pupils achieving level 4 in English and maths.
A total of 1,486 state secondaries - 48.6 per cent - were below average on the main five A*-C GCSE including English and maths measure.
The guidance for inspectors piloting the new framework shows that any of these schools not deemed to be "closing the gap" will be given an "inadequate" grade for achievement, which in turn leads to an overall "inadequate" rating.
An Ofsted spokesman said: "Lower-than-average attainment on its own would not result in an inadequate judgment for achievement, as long as there is evidence of an improving trend and pupils are making good progress from the time they joined the school."
Original headline: Thousands of schools could fall foul of new Ofsted regime