Ofsted is refusing to release details of a new inspections regime being introduced in September that is expected to place further pressure on schools in tough areas with low- achieving pupils.
Headteachers' leaders say the delay is causing unnecessary anxiety and will lead to schools becoming "victims" of the new inspection process.
The watchdog has indicated that the judgment inspectors reach on pupil achievement will no longer just be based on the value added, but will place a greater emphasis on raw exam scores that take no account of pupil background or prior attainment.
But despite repeated requests from The TES and teaching unions, Ofsted will not say how this will work and what the balance between raw and value added scores will be.
The watchdog says it wants to take account of pilot inspections continuing this term before discussing the detail.
But The TES has learnt that some inspectors have alrready been trained in the new process.
Keith Dennis, Association of School and College Leaders inspections consultant, said: "In schools likely to be inspected next academic year there are high levels of anxiety because they do not know the criteria on which they will be judged.
"School leaders will effectively be turned into victims in the inspection process because they don't have the information that inspectors have."
The change will be crucial for schools because the grade they receive on pupil achievement will have a direct bearing on their overall verdict.
Ofsted says an "inadequate" achievement grade will mean the overall grade is "most unlikely" to be better than "satisfactory" and is "likely" to be "inadequate".
It follows the news of a new performance measure likely to further penalise schools in tough areas with low overall raw exam results.
A reformed contextual valued added (CVA) measure will give schools extra points for performance in English and maths.
According to a government document revealed by The TES last month, it will push most secondaries with low, unadjusted, raw GCSE results down the CVA league table.
Mr Dennis said the new inspections could hit schools facing the biggest challenges the hardest, making staff recruitment more difficult.
An Ofsted consultation last year showed teachers believed that judging on raw exam results "would be unfair to schools working in the most challenging contexts, that it would penalise weaker or more vulnerable learners".
Mick Brookes, general secretary of the National Association of Head Teachers, echoed their fears this week, saying he hoped it would not lead to inspectors saying "the toughest school in Bradford is worse than the most advantaged in Surrey without taking their backgrounds into account".
But an Ofsted evaluation of pilot inspections that took place in the autumn term found that although some schools were uneasy about the greater emphasis on raw results, the "great majority" said the weighting they were given was realistic.
Asked for details of the weighting, a spokesman for the watchdog said: "It would not be appropriate to discuss in detail isolated sections of an evaluation schedule which is still developing and changing."
He said they would be announced "later this term".