Inspectors have proposed that successful schools put GCSE-age pupils in for AS exams to improve their value-added results.
But heads have rejected the suggestion, saying they are unwilling to put 15 and 16-year-olds under such pressure.
Heads of successful schools say it is impossible to get top value-added scores because their pupils have such strong prior attainment.
Iain Blaikie of Alcester grammar school is one of five south Warwickshire heads who wrote to Ofsted to complain about its value-added measure, introduced last year. But he rejects the idea of boosting scores by getting pupils to do AS-levels.
"We could put all our GCSE students in for one AS as well, but they are not guinea pigs or cannon fodder," Mr Blaikie said. "Many are doing 11 or more GCSEs already, they are under enough pressure, and an AS would break them."
Mr Blaikie said that his school would never get an "A" rating for value-added even though 100 per cent of students get nine A*-to-C grades.
The school would not even get credit for adding value with pupils who got eight A*s at GCSE.
David Bell, then chief inspector, replied shortly before he left Ofsted in December. He disagreed with the heads' claims that the system was arbitrary and created a "ceiling for achievement". "A result of 8 A* (GCSE grades) is not the 'ceiling' in terms of the maximum number of points that can be scored by a pupil because some pupils take AS exams," he said. "For the vast majority of your pupils there is scope to exceed the score predicted."
John Dunford, general secretary of the Association of School and College Leaders, said other heads had been told by inspectors that they should be putting GCSE-age pupils in for AS.
"It is a perverse incentive," he said. "There are many more educationally stimulating things which 16-year-olds could be doing instead of sitting more exams."