( BBC ) - Increased online marking of exam papers could be linked to rising numbers of inaccurate grades, the National Association of Head Teachers says.
The association is collecting evidence of inaccurate results and demands for papers to be marked again.
In England, 40% of A-level papers were marked online. One board, Edexcel, says all its papers will be next year.
But the Qualifications and Curriculum Authority said technology should make assessment easier.
Teachers at Tolworth School, in Surrey, told the BBC they had sent back unprecedented numbers of papers to be re-marked this year as they were shocked at the grades many A-level and GCSE students received.
Pattern
Student Katie Davison thinks she was affected.
She said: "I got two As and a D in the third paper which was completely unexpected and I was not happy with it at all but it means that I can't go to university with that D grade."
Clarissa Williams, the school's head teacher and a member of the National Association of Head Teachers, said: "Talking to my colleagues at the NAHT, there are many schools where there are problems; there seems to be a pattern. My own association is gathering that evidence right now."
Former exam marker Margaret Tomkins told the BBC she had found papers could not be marked online with "integrity", and so she had resigned.
John Bangs, head of education at the National Union of Teachers, said legitimate questions had been raised and he was concerned.
"The information is that examiners are finding online marking quite tough.
"They are not able to annotate the scripts by hand, there's a time constraint and you can't take into account youngsters who do quite a lot of writing and don't fill in the standard box that online marking demands.
"So legitimately there's a question whether or not online marking is missing some of the achievements of youngsters."
The use of technology should not diminish the professionalism of teachers in the exam process, he added.
But the Qualifications and Curriculum Authority, which oversees the exams, says online marking is not a problem.
Isabel Nisbet, director of regulations and standards at the QCA, said: "We are about making sure assessment is good - that it's properly done, it's properly checked, it's properly supervised and actually technology should make that easier.
"The days of soggy parcels outside the teachers' houses have gone and it should be a way to support much greater supervision and checking."
She said online marking supported good quality marking and provided feedback for examiners, and as much care went into the process as on paper.
Ms Nisbet admitted some online systems prompted the examiner to move on to the next question after a certain time period, to avoid a backlog of papers to mark.
But there were also "quality alerts" provided by a supervisor monitoring the examiner at work.
Former chief inspector of schools, Chris Woodhead, said online marking was partly responsible for inaccuracies in grades, but at the root of the problem was a shortage of experienced, qualified examiners and their pay was too low.