Heads are being advised that it is no longer worth their while complaining or appealing to an independent adjudicator about Ofsted inspections, The TES has learnt.
Union experts say changes to the way appeals against the watchdog are handled mean schools now lack any effective way of challenging Ofsted on judgments they believe are unfair.
The news comes in the week that Ofsted confirmed the stark consequences of its new regime for schools, revealed by The TES last week.
The official figures, which cover a shorter period than The TES looked at, paint an even bleaker picture, showing that the proportion of schools judged inadequate more than doubled to one in ten.
John Dunford, Association of School and College Leaders (ASCL) general secretary, said: "Excessive reliance on data and raw attainment have no doubt been the cause of the increase in schools being put into categories (judged inadequate)."
But the union says those schools no longer have a worthwhile means of redress following last year's decision to transfer the duties of Elizabeth Derrington, the independent complaints adjudicator for Ofsted, to an outsourced service.
Keith Dennis, ASCL inspections consultant, said: "I now say to schools, `I don't think it is worth your time'. Now you just get some feedback saying Ofsted has followed its complaints procedure. It is another factor that adds to the dismay with which people view Ofsted."
Schools can apply to the independent adjudication service if they are unhappy with the way Ofsted has handled a complaint or appeal.
Ofsted said the Government's decision to award the adjudication contract to the Centre for Effective Dispute Resolution had speeded up the service but not changed its role.
But Mr Dennis said that although Ms Derrington, who served for eight years, had no power to overturn inspection judgments, she sometimes used her position to recommend a quick re-inspection.
That no longer happened because the remit was interpreted so tightly, he said.
"The Ofsted complaints people have a look at the sheets the inspectors have filled out and say, `Does the evidence on this sheet back up the judgment?' and of course it will do," he said. "But often the complaint is about how the evidence is gathered."
An Ofsted spokesperson said: "There is nothing to prevent a parent or school asking Ofsted to bring forward the inspection if they believe something has gone wrong in a previous inspection, and Ofsted would give this thorough consideration.
The watchdog's figures for the autumn term showed its new-style inspections had pushed up the proportion of inadequate schools from 4 per cent in 200809 to 10 per cent, with the proportion of secondaries placed in the bottom category more than tripling, from 3 per cent to 10 per cent.
As expected, Ofsted said the sample was skewed because it had focused more on weaker schools.
But it did not quantify how much difference this focus had made and admitted the change was also due to raised "expectations".
The NUT said Ofsted had inspected the full range of schools. Christine Blower, the union's general secretary, said the watchdog had "driven many good schools to the wall for no valid reason".
Christine Gilbert, chief schools inspector, said unions' comments did not match the views of schools Ofsted surveyed, with 90 per cent agreeing that the inspection judgments were "fair and accurate".
Asked what she would say to heads who lost their jobs because of inspection verdicts, Ms Gilbert said: "My job is to raise expectations on behalf of the pupils and parents."
As The TES revealed last week, the proportion of schools rated "outstanding" fell from 19 per cent in 200809 to 9 per cent.
Original paper headline: Heads' union: appealing against Ofsted is `not worth the time'