Three out of four independent schools, including some of the best-known institutions in the country, had their GCSE results under-estimated in the league tables published this week.
The main reason for the distortion, says the Independent Schools Information Service (ISIS), is that many private schools have a number of "under age" and "over age" pupils in Year 11.
Pupils who take GCSEs early eventually have their results counted when they reach 15. But those who take them late never have them counted.
In its performance figures, the Department for Education and Employment counts 15-year-old pupils preparing to take GCSE the following year - a year later than normal - as candidates gaining no grades.
This especially affects boarding schools, which have higher proportions of pupils from overseas or pupils whose previous educational experience has been distorted. Nearly 100 boarding schools are shown with GCSE scores more than 20 per cent worse than they would have been if all their 1998 candidates had been included.
At Cobham Hall, a girls' boarding school in Kent, for instance, 23 girls were aged 15 last year but only 19 of them took GCSE. The school is therefore shown with an average points-per-candidate score of 38, compared with the 57. 3 its candidates achieved.
Many candidates in independent schools are also "under age" but their results are eventually counted when they reach 15.
Winchester College, one of the most academic schools in the country, appears with a ludicrously low score of 30.3 (against an actual 59.4 average), but for a different reason. Almost all its pupils take four of five International GCSE exams - which the school regards as more rigorous - but those results are not counted by the DFEE.
League tables, centre pages