Official figures being released next week are expected to show a significant drop in the proportion of pupils achieving the main GCSE performance measure for the first time since its introduction.
The predicted fall in students attaining five A*-C GCSEs including English and maths (5ACEM) is likely to result in a greater number of schools dipping below the government's minimum "floor" standard, putting headteachers' jobs at risk and schools under threat of intervention and ultimately closure. One local authority has already disclosed a drop of almost 9 percentage points in students achieving the benchmark.
The news comes in a week when prime minister David Cameron has piled more pressure on "failing" schools, unveiling a plan that would involve regional school commissioners being given new powers to "remove" headteachers.
Secondaries serving the most disadvantaged students, who qualify for the pupil premium, appear to have been hit hardest by this year's GCSE statistics, TES understands. Some headteachers are said to be in "despair" over the results.
The decline can be explained at least partly by changes to exam structures and the way the crucial 5ACEM measure is calculated, leading to fears over the validity of the next set of league tables. Anne Davis, principal of Matisse College at Longfield Academy in Kent, said: "This is the real problem with the way education is going. There are so many changes that how can you compare one year with the next?"
GCSE results in August showed a slight rise in the proportion of entries achieving A*-C grades, from 68.1 to 68.8 per cent. But, as qualifications regulator Ofqual warned at the time, the headline figure masked huge volatility and variation between schools. Statistics being published next Thursday will be the first indication of how this has affected the 5ACEM measure and are on course to give a very different picture to the figures released in the summer.
One local authority, Brighton and Hove, has revealed that it is expecting a sizable fall in pupils achieving the benchmark, from 64.7 to 56 per cent. A report from the council adds that contact with other local authorities shows that the "significant majority" are also reporting drops, with an average decline of 3 percentage points.
Several well-placed sources have told TES that they expect the drop in the national 5ACEM figure to be even bigger, representing a "step change" from previous years. Another local authority said privately that its figures were "catastrophic".
Last-minute decisions about exactly how new league table qualification rules should be applied mean that the government still has limited room for manoeuvre, but it seems highly likely that there will be a significant break from the past. Department for Education statistics for the 5ACEM measure go back to 1995-96 and show that the national percentage of pupils achieving the benchmark has risen every year since, apart from 2002-03 and 2012-13, which both had tiny 0.2 percentage point drops.
Sir John Rowling, chair of the PiXL (Partners in Excellence) Club - a group including more than 1,000 secondaries - said that having seen members' figures he would be "staggered" if there was not a drop of more than 4 percentage points on the benchmark this year.
Part of the explanation was the government's September 2013 decision to allow only a pupil's first entry in a GCSE, rather than their best result, to count towards league tables, he said.
Sir John added that a new rule limiting the worth of each vocational qualification to a single GCSE in the league tables was another factor. "This is like comparing your golf score on a bad day with your golf score on a sunny day, especially if the holes are made smaller and somebody's nicked half your clubs," he said. "It is bound to be a different score. People are desperate."
Sir John said he knew of headteachers who were "on the edge" because of last summer's results. He added: "Their nervous systems must be nearly wrecked by this."
A move from modular to linear GCSEs, widespread school concerns over the grading of English and hundreds of secondaries reporting unexpectedly low results in maths may also explain the decline on the key GCSE measure.
Mary Bousted, general secretary of the ATL teaching union, said she thought schools serving disadvantaged pupils had been particularly hard hit by Ofqual's decision to remove speaking and listening from the final English GCSE grades.
"The effects have been catastrophic for many," Dr Bousted said. "In this quest for the perfect exam we are cutting out the things which are essential for young people."
`Headteachers have lost faith'
Ros McMullen, executive principal of David Young Community Academy in Leeds, describes her school's most recent GCSE results as "bizarre".
Lower than expected science grades will lead to a drop on the headline 5ACEM measure to 40 per cent, meaning that the school has only just met the government floor target.
Ms McMullen says the academy's "best ever" results in English and maths will keep the pressure at bay, but she is concerned about colleagues and schools elsewhere.
"It is very distressing when headteachers don't have confidence in examination results and grading," she says. "And that is where we are at.
"There is so much volatility and so many bizarre situations. Headteachers have lost faith.
"Because most of the fiddling with grade boundaries has been done around the C-D borderline, it appears to be schools serving the most disadvantaged communities that have been disproportionately hit.
"That seems quite a stupid thing to do, because if those schools are improving but you keep pulling the ladder up then you are not going to be able to measure the extent of their improvement."