Headteachers demanded an immediate investigation last night after a senior examiner claimed GCSE results were being "fixed" by deliberately reducing the pass mark.
Jeffrey Robinson, who has just retired after 16 years as one of the principal examiners for maths with the Cambridge exam board – now the Oxford, Cambridge and RSA board (OCR) – said grade boundaries were repeatedly lowered in the 1990s to achieve better results.
His comments drew a chorus of denials from his former employers and the Government's exams watchdog, the Qualifications and Curriculum Authority (QCA).
Mr Robinson insisted: "It's not opinion. It's fact. The levels at which the thresholds are drawn between grades have come down very significantly. They are effectively as much as 50 per cent lower in the case of C-grades than they were in 1988. Everybody is happy because the results get better, but people who took the exam 12 years ago and got a grade C may be a little miffed."
He said candidates in 1989 who took the intermediate-level paper set by the OCR board's predecessor would have had to score 65 per cent to get a C-grade. That had been reduced to 45 per cent last year. The grade boundary for one higher-level paper had been reduced from 48 per cent in 1989 to 18 per cent, he added.
Mr Robinson said he had no evidence of political pressure being put on the boards to reduce grade boundaries. But he added: "There must have been something that said, 'All right, let's get good results', but it's got to stop sooner or later because we can't get any lower."
Last night, David Hart, the general secretary of the National Association of Head Teachers, called on ministers to instruct the exam board to produce an immediate report on Mr Robinson's allegations.
"I think his allegations are outrageous and a real kick in the teeth for students and their teachers over what they have achieved today," he said. "However, it is important that the examining groups do produce a report ... otherwise the whole exam system will be devalued."
The QCA denied the claims in a statement. "QCA has a regular programme of monitoring to ensure that standards in all examinations are maintained over time," it said. "There is no evidence that maths standards have declined. Comparing pass marks in maths in this way is highly misleading because the examination has been made more rigorous since GCSEs were first set in 1988."
Dr Ron McLone, the chief executive of OCR, said Mr Robinson had been one of 27 principal examiners for maths under contract for GCSE.
"It is always a shame when retiring examiners vent their spleen in this way and fail to apply the rigorous standards of argument they expect of their students," he said.
"Nearly 50 years ago only one person ran the mile in under four minutes. Today nearly all serious milers can do so – but the mile is still a mile. In the same way, it would be strange if more students were not doing better at A-level and GCSE."
The higher-level paper involved harder questions, he said. "Low marks on difficult questions get the same grade as high marks on easier questions."
A spokeswoman for the Department for Education and Skills said: "We wholeheartedly reject this claim, which is a slur on the achievements of young people. There is no evidence to support Mr Robinson." Ofsted, the government's education standards watchdog, and the QCA had closely monitored standards for the past 10 years, she said.
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