Some private schools are forcing less bright pupils to drop subjects to boost their exam league table position, headteachers have said.
The admission came as members of the HMC rounded on the Government and called for reform of the way exam results are published. In a paper presented to this week's conference, it was suggested HMC schools should ditch league tables because the lists "encourage targeting of particular pupils in the run-up to examinations to the detriment of others".
It added that there was a "practice of requiring marginal pupils to drop examination subjects for the benefit of the school".
Dr Martin Stephen, chairman of the HMC, whose own school, St Paul's in London, came top of the tables this year, said tables introduced "competition of the wrong type". "We are not sure they always encourage good practice on behalf of the pupils," he said John Dunford, general secretary of the Secondary Heads Association, which represents state and private heads, said: "Headteachers are forced to make perverse decisions... It is the interests of the pupils which should decide whether a subject should be given up - and not the league tables."
Last year the HMC called for rankings to be axed, although schools failed to go through with a threat to withhold information. This week its heads again voted "overwhelmingly" against the tables but said they had to co-operate with them in their present form.
However, it was claimed exam reforms will eventually make the tables redundant. The Government's 14-19 inquiry, led by former chief inspector Mike Tomlinson, is expected to call for GCSEs and A-levels to be replaced by a European-style baccalaureate when it reports next month.
Dr Stephen said: "We think the whole issue will need to be looked at very carefully when Tomlinson's proposals emerge, because we have a sneaky feeling Tomlinson may change the rules."
Instead of the current tables, HMC members want exam data to be presented in a "profile" document covering a wide range of information. It would include the aims of the school, type of intake and value-added measures.