Ministers' programme of turning low-performing schools into academies is leading to pupils being "spoon-fed" a narrowed curriculum as teachers do everything possible to improve results, heads have warned.
They say schools are desperate to avoid being converted - a move often combined with the removal of heads and senior teachers - as ministers implement the policy in their quest to "drive up standards".
Brian Lightman, general secretary of the Association of School and College Leaders, said the policy was triggering more teaching to the test and even "teaching to the grade" as schools focused on crucial borderline CD pupils.
This summer the government's "floor standard" for secondaries will rise from 35 to 40 per cent of pupils achieving five or more A*-C GCSEs including English and maths. Ministers want schools that consistently fail to meet that target to become sponsored academies.
Amanda Thain, head of Levenshulme High in Manchester, said she knows of heads with low results who are narrowing their curriculum in a bid to escape that fate.
"Where I am, there are a lot of schools just about keeping out of the position where they will be forced into becoming an academy, and that is a huge incentive for heads to spoon-feed and cajole and take students out of different activities that allow breadth and creativity," she said.
Mr Lightman agreed: "That is exactly what is happening. The schools in that situation have to focus relentlessly on meeting these indicators and that means all the broader aspects of the curriculum that employers value and are so important take second place. They will teach to the test. They will teach to the criteria of GCSE grade C."
Ms Thain said the country needed creativity, but curriculum time was "being squeezed in order to achieve government targets". She was speaking at a Pearson education event where Durham University's Professor Robert Coe said there was a "mismatch" between what A levels and GCSEs rewarded and what people actually wanted from education.
"My view is that we are at a really low point here," the assessment expert said. He argued that the exams and the current school accountability system were "a very toxic combination".
Ms Thain said that as a former vice-principal of Burlington Danes Academy in West London - part of the Ark chain - she had nothing against academies. But she was concerned that some academies were taking over underperforming schools that were not "the right match".
"There is a situation where a school is currently being taken over by a successful academy from a different catchment area," she said. "I have a serious doubt about how that school can understand the issues of a school that has huge problems because of its intake. It has meant there are other schools scrambling just to get every point they can to keep themselves out of it and to avoid something unsuitable being imposed on them."
A Department for Education spokesperson said: "We believe that the strong external challenge and support of an academy sponsor is the best way to improve schools that we find to be consistently underperforming... Wherever possible we want to find solutions that everyone can agree on - as we've done with many schools that have become academies. But ultimately this is about rooting out underperformance and driving up standards, so that students reach their academic potential."