A leading exam board has said all boards raised their grade boundaries for GCSE English this summer amid claims pupils have been marked too harshly.
AQA, the board with the highest market share for English, insists this year's grades were comparable to any other.
Many teachers say pupils were marked too harshly, with many unexpectedly failing to get a benchmark C.
The Association of School and College Leaders called for an investigation.
Some head teachers have said some pupils have been awarded a whole grade lower than they were predicted.
English teachers' anger
In GCSE English, 63.9% of entries got at least a grade C, compared to 65.4% last summer. Some 15% were awarded an A or A*, down from 16.8% in 2011.
In English literature, 76.3% of exams were awarded A*-C, compared to 78.4% last year, and 23.2% got at least an A, against 25% in 2011.
In a statement, the AQA said: "This summer, all the exam boards raised their grade boundaries for GCSE English in order to maintain standards. In AQA's case, this was by between 0 and 3 marks."
A paper on the Ofqual website says there can be as little as 8 marks between grades at the lower end.
The AQA statement said pupils often took units for a qualification in different exam series.
"We take account of how students have performed in each exam series when we set grade boundaries, in order to ensure that standards are maintained.
"While grade boundaries can therefore vary between exam series, students can be confident that the grade they get for an overall qualification one year would be the same the next."
Brian Lightman, general secretary of ASCL, has called for an investigation into English results.
He said: "The big issue, schools are telling us, is at the C/D borderline in English.
"What appears to have happened is that, halfway through the year, it was decided that too many students were going to get a C grade in English and the grade boundaries of the exam were pushed up very substantially.
"Students who were working at a C level throughout the year, who were told on their assessments that they were in line for a C, have found out today that this is worth a D. This means they may not get their places at college and sixth form.
"It is morally wrong to manipulate exam grades in this way - you are playing with young people's futures."
And concerns over English grades have prompted particular concern in conversations on social networking sites.
Dozens of teachers have expressed their anger on the Times Educational Supplement's website.
One said: "Have seen a very significant drop in results after 10 years of bringing in between 70 and 80%.
"Bitterly disappointing for all concerned, but particularly for the pupils who will receive results tomorrow."
Another wrote: "Our results have been decimated. We're 10% lower than last year.
"It does seem that our expected Cs became Ds and, because we're a school where most of our students are clustered around that C/D borderline, we've been hit hard."
'Comfortable' with grades
But Mark Dawe, chief executive of the OCR exam board, said the standard of its English qualification had been maintained.
"Boundaries do move, unit to unit, they do move, session to session, but the overall judgment of the examiners and the quality of the students' work is key here, and those boundaries sometimes have to move to ensure that's delivered."
Mr Dawe said examiners were "very comfortable with the overall grades they've been awarding to those students".
NUT general secretary Christine Blower said: "Shifting the goal posts for grades in particular the C/D boundary has had a huge impact on individual students and the future of schools.
"It is not only very unsettling but also extremely irresponsible. These are arbitrary changes which in no way reflect the work of students and teachers and are clearly unfair."
The concern over English results is pertinent because this year, schools in England have to ensure that 40% of their pupils reach the government benchmark of five A*-C GCSEs, including maths and English.
This is a tougher than last year's requirement, where school were expected to ensure 35% of pupils made this grade - in total 107 schools failed.
This is one of the measures used to decide whether schools should be taken out of local authority control and pushed into academy status. The other two measures relate to pupils' progress.