Schools with "good" English departments have seen a drop in their GCSE results in the subject this year, according to a headteachers' union.
This summer, reformed GCSEs in English language, English literature and maths were sat by thousands of pupils.
The new GCSEs have been awarded under a new numerical grading system – from 9 to 1 instead of A* to C.
The Association of School and College Leaders has been contacted about "unpredictable" English results at some secondary schools.
Geoff Barton, general secretary of the ASCL, said: "We are picking up stories of schools with good, well-established English departments that get good results every year [that have] seen a drop in their results.
“You have got schools reporting very unpredictable results, with some students getting a grade 3 in English whereas their other grades are much higher."
Earlier in the summer, exams regulator Ofqual warned schools to prepare for “more variability” in results following the introduction of the new GCSEs this year.
'Don't jump to any conclusions'
Mr Barton added: “It was always predicted that national standards would remain the same but individual standards would drop in some schools.
“But, of course, if that happens on your watch it is very hard not to take it personally. We just want to keep reassuring those school leaders and governors not to jump to any conclusions.
“We spoke to Ofsted yesterday about the importance of their inspectors not jumping to conclusions if they are coming into a school after potentially a year where there has been a dip in results."
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Suzanne O’Farrell, a curriculum and assessment specialist at the ASCL, said they have seen some schools reporting "swings in results".
"We are seeing schools that might have got As across the board in their unreformed GCSEs – and 7s in English and maths – and then students have got 3s in English," she said.
Mr Barton added: “We haven’t yet been able to drill into whether there was a particular issue. But we have been told from Ofqual there was no issue with English.
“But I think it is going to lead to an increase in the requests for re-marks because I think people are going to need to drill into it.”
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