Schools across the country are accusing an exam board of artificially deflating their pupils' AS-level grades to prevent a new version of an exam from looking too easy.
More than 20 secondaries have contacted The TES or posted on the internet with similar stories about the performance unit of the Edexcel drama AS-level.
They say pupils' prospects are being jeopardised with grades up to four levels lower than experienced teachers had expected. In one case a predicted B-grade pupil ended up with a U for the unit. One angry teacher is calling for a "mass rebellion" against Edexcel.
But the board said the requirements as well as the structure of its new two-module drama AS-level were different to the three-module exam that drama teachers had become used to.
An Edexcel spokesperson said all marking and moderation was in line with exam regulator Ofqual's code of practice and that although a "small number" of schools had been disappointed, many were pleased with their results.
At Westlands School in Torquay, nine pupils took the exam and were all predicted A-C grades. But one ended up with a U, two received Es and three Ds.
Mike Stewart, the head, has written to Edexcel pointing out that his pupils have never received a U grade for the unit before.
He has accused the board of shifting the grade boundaries and is now considering using another awarding body.
"It seems to me that it has changed the rules on this exam without telling anybody, in an underhand way, and that is no way to carry on," Mr Stewart told The TES.
Westlands is one of several schools that have been further angered because Edexcel has yet to provide teachers with a breakdown of the marks. Many schools believe the unexpected low grades are the result of changes to the structure of AS-levels this year that saw the removal of a written unit.
One teacher, who said they examined the performance unit - unit 2 - for Edexcel, wrote on The TES website: "We examiners were given much harsher criteria for marking the unit 2 performances this year, and we were told (reasonably enough) that this was to compensate for the lack of the old unit 3 written exam, which generally would bring down the overall mark.
"Without it, and using the old marking criteria, grades would have unrealistically gone through the roof. However, we were told that, although marking would be harsher, grade boundaries would be more lenient, so that the overall grades achieved would be on a par with the old spec. To my mind, this has clearly not happened."
Edexcel is extending its deadline for inquiries about the results for two weeks and has said it will provide all schools with further information about how their pupils performed.