Concerned teachers have suggested that GCSE English exams were marked too harshly this year after schools reported an unprecedented number of fails among their pupils.
Around 600,000 teenagers in England, Wales and Northern Ireland will be waking up to their GCSE results.
But English teachers who were shown early breakdowns of the marks complained that exam boards had substantially increased grade boundaries, leaving pupils with lower results than expected. It comes amid predictions that the pass rate across all subjects will start to stall.
The Association of School and College Leaders (ASCL) said there seemed to be a particular problem with C/D borderline grades in English, with pupils who were expecting Cs ending up with Ds.
ASCL general secretary Brian Lightman said there was "a lot of concern" among members about the changes to grade boundaries. He said: "We expected results to plateau because of the Government taking steps to peg them to previous levels.
"But we certainly didn't expect anything like this and schools have been taken by surprise. We've got some schools where teachers have been teaching the same course for years and suddenly the results are way below what they've always been before as the pass mark has been moved higher."
Dozens of teachers voiced their concerns about the issue on the Times Educational Supplement's website on Wednesday night. One said that the score out of 80 needed to get a C grade was now a whole 10 marks higher than earlier in the year. He added: "They've not moved the goal posts, they've put them on a different chuffing planet."
Another said: "Our results have been decimated. We're 10% lower than last year. Members of my department are in a state of shock as they say they've never worked harder and this is the result."
An ASCL spokeswoman blamed the Government's "constant tinkering" with the exams system, adding: "They are fiddling and manipulating them all the time which is not fair for the kids who have worked really hard and have been working at a C grade level but now may not have got one after all."
Last year 69.8% of GCSE entries gained at least a C grade, and 23.2% got an A or A* but one expert predicted the pass rate will stall this year.