Headteacher Trevor Harding said Belper School had experienced an “unexplained dip” in its GCSE English results, as teachers across the country claim the exams were marked too harshly.
The amount of students achieving a benchmark C at Belper fell by around 10 per cent compared to last year.
Mr Harding said the drop in performance mirrored a national trend where 63.9 per cent of entries were awarded A*–C, compared to 65.4 per cent last summer.
“In common with many schools nationally we have have got an unexplained dip in English results,” he said.
“That’s a mystery to us at the moment. We have got something there we never had before .
“This is the same team of teachers that got an increase last year.
“It’s in the marking or the way that the exam is being assessed.”
Many teachers say pupils were marked too harshly and the Association of School and College Leaders has called for an investigation.
Some head teachers have said some of their pupils have been awarded a whole grade lower than they had been predicted.
The concern over English results is pertinent because this year, schools in England have to ensure that 40 percent reach the government benchmark C grade.
The Association of School and College Leaders (ASCL) said there seemed to be a particular problem with C/D borderline grades in English, with pupils who were expecting Cs ending up with Ds.
Belper School pupil Jonathan Shaw, 16, said: “I worked hard for my English exam and I was awarded a B when I think I deserved an A.
“I definitely think that the exam was marked too harshly in my opinion.”