This year Westminster School in London has beaten off the challenge from its long-standing rival, Winchester College, to come top of a table of provisional results for 520 fee-paying schools. Pupils at Westminster (boarding fees pounds 14,400 a year) scored an average of 33.46, more than three A grades each. The 650-pupil school takes around 150 boarders.
Tristram Jones-Parry, Westminster's head, who has been in the post for just a week, said he felt that the boarding school ethos encouraged success. "I think the atmosphere of a school that doesn't stop at four but continues for both day pupils and boarders helps.
"It means that the teachers are around after school so there are extra classes and people can be helped easily with their work."
Winchester, which was top last year when Westminster was second, is a boarding school, as is the fourth school in the table, Eton, where Prince Harry joins his older brother Prince William in the coming week.
St Paul's School in London, which came third, also has a small number of boarders. But the top all-girls' school in the list, Oxford High, which came fifth, is a day school. Westminster admits girls only in the sixth form.
Under the Universities and Colleges Admissions Service scoring system for A-level, an A gains 10 points and an E scores 2. The figures from the Independent Schools Information Service show that the average point score per candidate was 21.95 compared with 21.60 last year, a 1.6 per cent improvement.
The proportion of independent-school entries awarded grade A was 33.5 per cent, up from 31.7 the previous year;twice the national average of 16.8 per cent.
The list shows a huge variation in results, reflecting schools' very different intakes. While the top schools are highly selective, some independent schools cater for pupils of much lower ability.
Value-added league tables, which measure schools' performance against the ability of their pupils are to be introduced by independent schools for GCSE from 2000. Similar tables for A-level are likely to follow.
Mr Jones-Parry said: "If there are league tables, it is nice to be top but they do not necessarily tell you the quality of a school. We hope that Westminster maintains its position when value-added league tables are introduced."