Some of this year’s A-level grades have been “pulled down” in an “utterly unfair and unfathomable” way, heads have said.

The Association of School and College Leaders (ASCL) says it has received “heartbreaking feedback” from heads who are “dismayed” that Ofqual’s standardisation model has had a “devastating” impact on their students’ grades.

Geoff Barton, ASCL general secretary, said: “While there has been an overall increase in top grades, we are very concerned that this disguises a great deal of volatility among the results at school and student level.

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“We have received heartbreaking feedback from school leaders about grades being pulled down in a way that they feel to be utterly unfair and unfathomable. They are extremely concerned about the detrimental impact on their students.

“They worked very hard to provide accurate grades to the exam boards, carefully following all the guidance, and are dismayed that the statistical model then used to standardise these grades has had such a devastating impact.

“This is in terms of both the number of grades lowered, and some students’ results being pulled down by more than one grade.

“We will be working to understand more about what has happened, but our immediate impression is that the statistical process has proved to be far too blunt an instrument and has created clear injustices.”

He added: “We have always accepted that some form of standardisation was needed to provide consistency, and we recognise that this was never going to be easy. But the education system is not a statistical model, it is a collection of individuals, and we fear the process has lacked this important degree of nuance.

“We are now calling on the government and the exam regulator Ofqual to review the situation as a matter of urgency, and we would warn them against simply digging in their heels, and insisting all is well.

“It is not sufficient for the government to dismiss these concerns by saying that schools and colleges can attempt to battle their way through the appeals process, or that students who are not satisfied can enter the autumn exam series some seven to eight months after they finished their courses, and are no longer at the centre where they studied.

“We have done everything we possibly can to support the grading process in difficult circumstances, but there is a time to say enough is enough.”