More than 600,000 schoolchildren will open their GCSE results later amid controversy over the introduction of tougher exam standards.
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 (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.
"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."
Dozens of teachers voiced their concerns about the issue on the Times Educational Supplement's website.
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."
The tougher grading standards could have serious consequences for some schools.
This year, for the first time, all schools have been forced to ensure that 40% of pupils gain at least five good grades including in English and maths, up on the 35% target by which schools were judged last summer.
It is claimed that up to 250 schools could fail to hit the benchmark.
Ministers have warned that schools falling below national GSCE targets could be closed or pulled out of local authority control and turned into independent academies run by a third-party sponsor. Failing academies could undergo a change of leadership.
The Department for Education said the Government made "no apologies for setting high expectations and will not hesitate to step in where there is evidence of chronic underperformance".
Last year, 23.2% of exam papers were graded at least an A, the 23rd successive increase.
It is believed that efforts to stop grade inflation and a shift towards more rigorous academic subjects may lead to top-end scores stalling for the first time this year.
Mr Lightman added it was "morally wrong" to undermine teenagers' GCSE efforts with talk that the exams are too easy.
"I think we've got to be very careful with the message we send out about quality, it's potentially very dangerous to undermine confidence in the system," he said.
"The fact of the matter is children are working extremely hard and teachers are working extremely hard to get them through exams."