Four out of five schools said exam results were distorted because the Department for Education and Employment takes account only of those of pupils aged 15, ignoring any in the same year who are a different age. The survey showed that in some cases the Government's version of GCSE results was 20 per cent worse than the real figure.
Adrian Underwood, chairman of the association, said foreign parents were put off British schools which was losing the country millions of pounds.
In Mr Underwood's school for girl boarders - Moira House in Eastbourne - 88 per cent of Year 11 gained five or more A-C grades. The DFEE figure was 67 per cent. The association was sending its objections to the department.
Mr Underwood said seven younger and seven older girls' results were ignored. "Many independent schools are affected by this issue, but boarding schools suffer particularly as they have always offered educational opportunities to so-called 'out-of-age' pupils."