Around 600,000 students in England, Wales and Northern Ireland are waking up to their GCSE results on Thursday.
But, as they await their grades, concerned teachers have suggested that English exams were marked too harshly this year after schools reported an unprecedented number of fails among their pupils.
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.
The Association of School and College Leaders 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.
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.
Another wrote: "Members of my department are in a state of shock as they say they've never worked harder and this is the result."
Last year 69.8 per cent of GCSE entries gained at least a C grade, and 23.2 per cent got an A or A* but one expert predicted the pass rate will stall this year.