Teachers hit back today after the exams regulator claimed they were guilty of significantly over-marking papers amid pressure to produce good results.
Teenagers were let down this summer by an exam system that is abused by teachers, Ofqual said in a new report into the GCSE English fiasco.
Chief regulator Glenys Stacey laid blame for the debacle on intense pressure on schools to reach certain targets, which led to over-marking, as well as poorly designed exams and too much emphasis on work marked by teachers.
But teaching unions reacted angrily to the suggestion of deliberate over-marking, arguing that teachers should not be made scapegoats.
Russell Hobby, general secretary of the National Association of Head Teachers (NAHT), said that schools are judged on their pupils' performance in GCSE English, and called for a re-think on how schools are held accountable.
"It is a diversion to attempt to blame teachers for following the rules they were given," he said.
"If your elected government tells you this is the right thing to do, if your performance is measured on it and if you are sacked for failing to achieve it, you have no choice but to do it.
"Our punitive and single-minded focus on C grade exam passes does indeed deserve a share of the blame for this summer's re-grading fiasco. It amplified the effects.
"The way we hold schools accountable has distorted, degraded and at times corrupted our examination system.
"In particular, the crude focus on a single summary statistic (the percentage of five A*-Cs) has forced the profession to concentrate on hitting that benchmark because of the dire consequences of failing to achieve that magic number."
Teachers were "not to blame for the grading shambles surrounding the exam and they should not be made scapegoats for the system," the NAHT said.
Malcolm Trobe, deputy general secretary of the Association of School and College Leaders (ASCL), said it was "outrageous" to suggest that teachers and schools were to blame, adding that Ofqual is responsible for ensuring "fairness and accuracy" in the system.
Headteachers have said that tens of thousands of teenagers received lower GCSE English grades than expected this year, mainly around the C/D border, after exam boards moved the grade boundaries between January and June.
An initial report by Ofqual concluded that some of January's assessments were "graded generously" but the June boundaries were properly set and candidates' work properly graded.
Publishing a second report into the fiasco today, Ms Stacey said: "We have been shocked by what we have found. Children have been let down. That won't do," Ms Stacey said.
"It's clear that children are increasingly spending too much time jumping through hoops rather than learning the real skills they need in life. That won't do.
"Teachers feel under enormous pressure in English, more than in any other subject, and we have seen that too often, this is pushing them to the limit. That won't do either."
It was hard for teachers to maintain their integrity when they believe others are abusing the system, Ms Stacey said.
Mr Trobe said that the fact that different standards were applied to January and June exams was "blatantly wrong."
"The accountability measures do place tremendous pressure on teachers and schools, especially at GCSE grade C, but to say that teachers would compromise their integrity to the detriment of students is an insult."
Christine Blower, general secretary of the National Union of Teachers (NUT), accused the regulator of "shifting the blame whilst at the same time exposing the nonsense of floor targets."
"The fact remains that young people were let down," she said.
"The solution is to regrade the exams of young people who, together with their teachers, worked to the parameters set in January."
An alliance of pupils, schools, councils and professional bodies have launched a legal challenge over the fiasco, calling for this summer's English results to be re-graded.
Chris Keates, general secretary of the NASUWT teaching union said: "Ofqual's report has highlighted the poisonous relationship between the qualifications system and the accountability regime. This is at the heart of the controversy.
"The accountability system is not fit for purpose and places unacceptable pressure on schools. If it were fit for purpose we would not be having this debate.
"There is now a window of opportunity for the Secretary of State to issue the long overdue consultation on school accountability and put forward his proposals for addressing the issues raised in Ofqual's report."
The new English GCSEs, which were awarded for the first time this year, were split up into modules, with pupils sitting written exam papers and "controlled assessment" - coursework completed under strict classroom supervision.
It was down to schools to decide when pupils submitted their controlled assessment work and sat the exams.
Ofqual's report found that many schools used the marks pupils received in their first exams and the January grade boundaries to work out what score a pupil would need in their controlled assessment and marked it accordingly.
The majority of controlled assessment work was submitted in the summer, and examiners saw evidence of over-marking.
As a result, grade boundaries were raised to take account of this, and led to some students getting lower grades than expected.
Ms Stacey said the distribution of this year's GCSE English results, which saw bunching around the C grade boundary, was "shocking".
"The unexpected pattern, the unprecedented clustering around perceived grade boundaries for each of the whole qualification is striking."
This was "not simply aspirational marking" by paper or unit but a "targeted approach to the whole GCSE", she said.
"It is very hard for teachers to maintain their own integrity when they believe there is widespread loss of integrity elsewhere," Ms Stacey said.
Ofqual had spoken to teachers who said they believed that "teachers elsewhere were abusing the system", she added.
"No teacher should be forced to choose between their principles on the one hand and their students, their school and their career on the other."
Under the current system, schools are judged on the number of pupils who score at least five C grades at GCSE, including English and maths.
This measure is included in league tables, with schools expected to have at least 40% of students reaching this standard.
Those which do not, and fall short on other pupil progress measures, are considered failing.
The report says the new qualifications "reinforced the trend" of schools running the GCSE schools years (Years 10 and 11) as a "tactical operation to secure certain grades and combinations of grades".
"This has come to be seen as 'what good schools do' despite the awareness of many teachers and parents that the concept of broad and deep learning can get lost along the way."
Ms Stacey said their investigation had told them there were "special pressures on this subject and the fact that this GCSE is designed in such a way that it is susceptible to these pressures".
Other subjects do not carry the same pressures as this "one key central measure, C grade English".
Mr Stacey later added that it was "appalling really for students to be wrapped up in anything like this".
From September next year, English GCSEs will no longer be modular in England, she said.
Any exams or work submitted next January will be marked, but not graded until after June's exam season.
Moderation of the exams will also be tightened, she said, and exam boards will have to improve their communication with schools.
Ofqual will also talk to the Government about the findings as it is due to look into accountability, she said.
It will also advise on exam reform, Ofqual said, warning that care must be taken when introducing new exams such as the Government's new English Baccalaureate Certificate, which are set to replace GCSEs.
Asked about the effect on future qualifications, Ms Stacey said it was "early days" and it was not known yet what a new course would look like.
"Choices would have to be made," she said.
"If pressures stay as they are, qualifications will need to be locked down."
This would most likely mean more written exams and less controlled assessment, as well as different systems for marking.
It has been estimated by the Association of School and College Leaders (ASCL) that hundreds of schools saw a large fall in the numbers of pupils scoring at least a C in GCSE English this year.