Official figures being published next week are expected to reveal a significant drop in the proportion of pupils achieving the main GCSE performance measure for the first time since its introduction.
Schools say the fall in the percentage of students passing five A*-C GCSEs including English and maths has been particularly pronounced among disadvantaged students who qualify for the pupil premium.
One local authority has revealed a drop of nearly 9 percentage points on the measure and another described results in its area as “catastrophic”.
Some headteachers are said to be in “despair” about the results, which could trigger government intervention and potentially the closure of low-performing secondaries.
They are blaming government-directed changes to the structure of exams and the way that league tables are calculated for the fall and warn that the next set of league tables will not present a fair comparison with previous years.
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