The biggest secondary headteachers' union has backed Ofqual's decision to investigate potential discrepancies in last year's A-level maths grading, describing it as a matter of "natural justice".

This morning Tes revealed that Ofqual is investigating grade boundaries set by exam boards last year for students who took the reformed A-level maths qualification after one year of study.

About 2,000 students took the reformed qualification in 2018 – those that did so were mainly very able 17-year-olds planning to take further maths in 2019.

But for this year’s exam – which was sat by 85,000 candidates in England – grade boundaries were set much lower than in 2018.

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Ofqual believes that the 2019 grade boundaries are “sound” but it is now investigating whether the boundaries in 2018 were too harsh.

Now Geoff Barton, Association of School and College Leaders' general secretary, has told Tes that Ofqual was right to act. “It demonstrates that a regulator is doing what a regulator ought to do," he said.

"If you’re going to have faith that standards are being maintained, not just across a year and across exam boards but between years, then the only way you can do that is to have a look at what have we learned this year from a big cohort that we didn’t know last year with a small cohort.

"You can either say well let’s just learn that and do nothing about it, or if you think there’s a sense of natural justice – which it sounds like Ofqual do – then you would have to make amends for it."

Meanwhile another teachers' leader has warned that doubts over whether some pupils sitting A-level maths last year got the right grades undermines “integrity and confidence” in the entire exam system.

Mary Bousted, National Education Union general secretary, said revelations that hundreds of students could have their grades upgraded showed that the education system was overly reliant on written exams.

She said that no one expected grading to be a “completely exact science”, but added: “What is worrying is that at present it seems to be too inexact a science. And it’s a science upon which young people’s futures depend.

“We need a more rounded assessment system where there is space for other forms of assessment which would balance out the known unreliability of exam grades.

“When you get examples like this – although Ofqual is doing the right thing because it’s ensuring consistency of exam marking – it does undermine integrity and confidence in the whole thing.”

Ofqual's investigation could result in some students getting their grades retrospectively changed, but many will already have applied to university on the basis of the A-level grades they already hold.

Mr Barton said there would be “frustration” among heads and students that incorrect grades could have been awarded.

The Joint Council for Qualifications, which represents the exam boards, insisted that grading had been correct.

In a statement it said: “The grade boundaries were set according to Ofqual’s procedures and approved by them at the time, and students and teachers can be confident in the grade boundaries set.

“Over the last week or so, the awarding bodies have been providing additional and detailed evidence which, on balance, confirms that the grade boundaries in 2018 were set correctly. We will continue to work and cooperate with Ofqual in any review of the grade boundaries.”