Inspection body Ofsted has been accused of being out of touch with the needs of schools following its release of inspection figures.
Half of the 2,140 schools it inspected last autumn under a new framework were labelled either satisfactory or inadequate.
The number of schools slammed as inadequate had more than doubled to 10 per cent compared with 4 per cent in 2008-9.
Chief inspector Christine Gilbert said the results were because Ofsted had raised expectations but teaching unions accused the body of "moving the goalposts."
Union NASUWT general secretary Chris Keates said: "It is the equivalent of preparing to play a cricket match and turning up to find you are expected to do the high jump.
"Ofsted is losing the confidence of schools, staff and parents by building an inspection system on constantly shifting sands."
The NUT contested a number of Ofsted's claims about the new inspection regime.
While Ofsted said it was focusing more on "weaker" schools, NUT general secretary Christine Blower said: "Evidence from our members shows that Ofsted have inspected a full range of schools and not just focused on 'weaker schools'."
Ofsted also said that inspectors spent twice as long monitoring lessons, but Ms Blower retorted that "many teachers have complained that they have been subjected to very short inspections and those snap-shot observations are being used to make less-than-informed observations about teaching and learning."
Ms Blower added: "The picture that Christine Gilbert paints of schools finding the new inspections regime a fair and positive experience bears no resemblance to the reality many teachers and head teachers are experiencing.
"The NUT has been inundated with teachers expressing concerns about punitive and unfair inspections led by inspectors who themselves often have little idea of life in the classroom.
"Where teachers do need some extra help in the classroom, it is not inspection which will improve their skills but support and training, which currently is not available to many teachers."
Earlier this year, NUT members staged a "no notice" protest outside the Ofsted headquarters in London. The action was triggered by teachers reporting that the new framework had led to unfair judgements about their work.
According to the union, one teacher was failed by inspectors because she left a door propped open.
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