Thousands of teenagers are being put in for multiple GCSE maths exams in the hope they will get crucial C grade passes in at least one of them.

The practice is exposed by the exams regulator Ofqual today as it reveals that 15 per cent of candidates sitting GCSEs – around 90,000 candidates – were last year submitted for maths exams with more than one board. Ofqual officials believe there will be a repeat this year because the pressures that drove schools to do it – including boosting performances in league tables – are still there.

Glenys Stacey, Ofqual’s chief executive, said there was a “fine balance” as to whether the practice was in the best interests of the pupil or was instead “demotivating them by putting them through a number of assessments”. “Time could be better spent preparing them for one assessment,” she added. This would allow them to move on to other subjects.

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The practice, which has come to light for the first time this year, is one of two methods being used by schools to try to ensure C grade passes in English and maths – crucial for determining a school’s ranking in exam league tables. In addition, they are putting growing numbers of candidates in for exams a year early – at 15 or 16 – so they can bank a C grade pass in English or maths and then drop them to concentrate on their weaker subjects the following year, with the insurance that, if they don’t do well, they get a second chance 12 months on. This year saw 23 per cent of the maths cohort sitting maths a year early compared to 18 per cent last year. In English literature and language, the figures rose from 7 and 8 per cent respectively to 11 per cent and 10 per cent. Dennis Opposs, of Ofqual, described the figures as “quite a huge increase in [early] entry”.

But the practice may in fact lower the percentage of top A* , A or B grade passes for pupils. “A year’s more maturity in maths is a significant thing,” said Mrs Stacey. However, she warned that some candidates may be taking an exam early “just to get it out of the way”, in some cases leading 15-year-olds who achieve a C grade pass to accept it and not find out whether the extra year would have improved their grade. She added pupils registered with two boards often drop the one with which they are having difficulty.

Ministers frown on the practice and last month sent a memorandum to the Commons Select Committee on Education saying they would ask Ofsted, the education standards watchdog, to look at the “inappropriate use” of multiple exam entries. The Education Secretary Michael Gove is also planning changes to league tables so that they no longer focus so much on the percentage of pupils getting five A* to C grade passes including maths and English.

Christine Blower, the general secretary of the National Union of Teachers, said: “When accusations fly that schools are somehow gaming the system, it is often the case a blind eye is turned to the malign influence of Ofsted benchmarks and ever-changing targets.”

She added: “Children, for whom teachers always want to do their best, develop at different rates. It is right, therefore, that teachers use their professional judgement to decide on early entry.”

Meanwhile Ofqual predicted “a small drop in achievement” in science as a result of the papers becoming harder to pass. On A-levels, Ofqual is expecting a rise in the take-up of subjects singled out by the Russell group of universities as “facilitators” that help pupils gain access to their universities. They include English, maths, history, geography and the sciences.