Heads are calling on Ofsted to explain figures that appear to show that the watchdog eased up on schools as the first three months of its tough new inspection regime progressed.
Official statistics released this week show that Ofsted judged 14 per cent of the schools it inspected in January and 12 per cent in February to be inadequate, but just 4 per cent in March.
Russell Hobby, general secretary of the NAHT heads' union, said the pronounced drop suggested "either an attempt by Ofsted to manage the impact (of high numbers of failing schools), or an inspection workforce which is only now becoming competent at applying the framework correctly".
He said that unless the inspectorate could provide a credible explanation, it should be open to re-examining the judgements of schools that were inspected early on in the framework.
Ofsted says such claims are "extremely misleading" and that the differences can be accounted for by some inspection reports from February and March still being moderated after the 1 May cut-off date for the statistics. But no mention was made of this when the figures were released and the watchdog was unable to provide full details of the number of inspection reports that were not finalised by 1 May.
Mr Hobby said the idea that moderation was the reason "just doesn't seem credible, it seems like a really weak argument". He added: "There will be people who have lost their jobs as a result of being failed in January and I think Ofsted will need to have an explanation as to why."
The proportion of primaries being failed fell steadily over the three months, from 15 per cent in January to 10 per cent in February and 5 per cent in March. The proportion of inadequate secondaries went from 19 to 21 to 3 per cent.
Reports are usually published a matter of weeks after an inspection. But more checks are made on inadequate verdicts, and challenges from schools can further lengthen the process.
Malcolm Trobe, deputy general secretary of the Association of School and College Leaders, said that under the new framework there had been a significant increase in the number of schools it was helping to challenge Ofsted judgements.
An Ofsted spokesman said: "It would be extremely misleading to conclude from today's official statistics that inspectors began the year with tougher inspection criteria.
"Inspectors are consistent, fair and objective. Inspection is based purely on evidence and inspectors do not change the way they interpret criteria by which they grade schools in the inspection framework." He added: "There are still schools that were inspected in the latter half of the first quarter of 2012 that remain in the moderation stage of the inspection process and do not appear in these provisional statistics."
Overall, the new inspection framework does appear tougher. Of the 1,964 state schools inspected between January and March, 7 per cent were judged outstanding, 50 per cent good, 34 per cent satisfactory and 9 per cent inadequate. The equivalent percentages in the 2010-11 academic year were 11, 46, 38 and 6 per cent.
The statistics also show that of the 144 outstanding schools inspected between January and March 2012, all had outstanding teaching. Concerns had been raised that a number of schools were being judged outstanding despite having teaching that was only good. Figures for the final quarter of 2011 show that 122 out of 294 outstanding schools had their teaching rated good.
Original headline: Heads demand to know if Ofsted is dropping tough act