The Brit school, in Croydon, south-east London, helped its 252 18-year-olds gain better than a B grade on average for each exam they took last summer.

The school is being seen as a pop star production line. Recent alumni include the indie band The Kooks, chart-topping singer Katie Melua, and the latest X-Factor winner Leona Lewis.

The school appears to have benefited from the widening of league tables this year to include more vocational qualifications.

Btec courses, which are popular with students but have never previously counted, are weighted in the tables as worth one, two or three A-levels, depending on which version the student takes.

Nick Williams, the Brit school’s principal, said 90 per cent of its post-16 entries last year were for vocational exams. The most popular were Btecs in subjects including performing arts, music, media and visual arts. Some 77 per cent of the school’s Btec entries gained a distinction last year.

Mr Williams said: “We always suspected that when vocational awards were included we would do well. This is great news.”

North London Collegiate, the private girls’ school in Edgware, north London, had the best results when judged by the average points score per A-level. Its average score amounted to almost an A grade for every exam taken by the 121 girls in its upper sixth.

National figures showed that, on average, students gained the equivalent of three B grades. The average performance per exam was between a D and C grade.

Truancy figures offered less pleasing news. Two secondaries, both to be replaced by academies - Minster college on the Isle of Sheppey, Kent, and Geoffrey Chaucer college in Southwark, south London - had the worst rates.

At Minster, 15 per cent of “half-days” were unauthorised absences. At Geoffrey Chaucer, the figure was 11 per cent. Third on the list was the Ridings school, in Halifax, with 10 per cent.