Headteachers blasted exam boards last night after a covert change in grade boundaries sent some English GCSE results plummeting and put youngsters' sixth-form places at risk.

As schools worked their way through their exam results they began to notice the grades were not living up to their expectations.

It has meant many students aiming for an all-important C – required by many sixth forms, colleges and employers – were left with a D instead.

Last night Sean O'Neill, headteacher at Bungay High School, described the situation as 'shameful', adding: 'They have changed the goalposts halfway through the course and penalised the young people.'

Ron Munson, headteacher at Taverham High School, said: 'Nothing we had beforehand had given us a clue the grade boundaries were changing. It's playing with people's education.

'It's an example of the exams being used as a political football and that's not fair. It's been a real surprise to many of my colleagues.'

The problems are thought to relate to a number of different exam boards and have affected schools across the country.

Some exam chiefs have admitted that the grade boundaries for a particular module were altered between the January 2012 assessment period and this summer's exams.

The exam boards, exams watchdog Ofqual, and the department for education have all insisted any changes would simply have been part of efforts to maintain standards and ensure the exams were 'robust and rigorous'.

Schools found their predictions for the percentage of students achieving A* to C grades in English were as much 15pc off the actual figure.

For many, it had a knock-on effect on the percentage of students reaching the government's 'gold standard' for GCSEs – at least five A* to C grades including English and maths.

Brian Conway, headteacher at Notre Dame High, in Norwich, said his school's headline figure was about 15pc below what he had expected because of plummeting English results.

Peter Devonish, headteacher at Neatherd High School in Dereham, said his English department had a good track record of accurately predicting outcomes for students and had been left 'dumbfounded' by yesterday's results.

He said the school had expected about 81pc of students to achieve A* to C grades in English but had found just 63pc reached that level – a 15pc drop on last year's results.

It means the percentage of students reaching the 'gold standard' at Neatherd High has dropped 13 percentage points.

He said: 'It's making a mockery of the whole system. These grades are important for the children and them being able to go out and get on with their lives.'

Many sixth forms require students to achieve a set number of C grades to study A-levels with them – with some even specifying a C in English as an entry requirement.

An increasing number of employers also require at least a C in English before they will offer someone a job.

'A C in English and a C in maths are now considered to be absolutely vital,' said Rob Anthony, associate headteacher at The Hewett School in Norwich. 'We have a number of students we were fairly certain were going to get a C and they have ended up with a D. They are now in a position where we have to question their places in the sixth form.'

Mr Anthony said he, along with many other headteachers across Norfolk, would take into account this year's problems when it came to offering sixth form places.

The problem with marking of English papers has not affected schools consistently across the board. While some schools have seen marked decreases in their English results, it has not made too much of an impression on their 'gold standard' figure, and some schools reported noticing very little change.

The grade boundary changes would have had a larger impact on those schools with more students on the C/D border.

Steffan Griffiths, headmaster at Norwich School, said he was not surprised to hear of yesterday's problems. This year's GCSE cohort received iGCSE results in English and English literature for the first time after the independent school chose to change which exams they took to get away from 'inconsistencies in marking'.

Mr Griffiths said: 'It certainly worked for us this year. We are pleased that we have not been affect this year but we have a lot of sympathy for the schools that have been.'

Last night a DfE spokesman said it was Ofqual's job to 'make sure standards are maintained over time' and added: 'There has been a widespread debate over the last two decades about whether there has been grade inflation - that's why we have strengthened Ofqual's powers to make sure the system is robust and rigorous and to give the public real confidence in the results.'