Secondaries are providing youngsters with detailed guidance, specifying what points should be made in each paragraph and which quotations to use, Britain's second-largest exam board said.
In its annual report on English GCSE, Edexcel highlighted as "insidiously worrying" the issue of "teaching by numbers", where the submissions of many pupils from a school were very similar.
"In such cases, teacher guidance to candidates stretches what is acceptable to the limit, by providing over-detailed essay plans."
The report warned that pupils would find it difficult to get higher grades, because it was clear their work was not original, and there was a high risk it would be penalised. At least 100 teachers were investigated by the boards for malpractice last year.