Girls schools dominate the independent schools A-level league tables this year, with City of London School for Girls achieving the highest results of any school yet.

The school's success, with 99.6 per cent of all grades at A and B, beats the previous record of 99.3 per cent set last year by North London Collegiate school.

Diana Vernon, the headmistress at City, said the school benefited greatly from having London on its doorstep, which led to fantastic extracurricular activities. She also attributed its success to its diversity - one in five girls receives financial support with fees. “The girls take the initiative; they spark off each other.”

She added that girls schools had an in-built advantage. “Boys hate coursework. They are possibly better at taking risks. But Alevels and GCSEs have removed a lot of the risk taking. We teach to the test and girls rise to the challenge. They jump through those hoops. Boys prefer to wing it a bit more which is probably why they do better at university,” she said.

At Cheltenham Ladies College, another school near the top of the tables, the headmistress, Vicky Tuck, attributes her pupils' success to “the culture of confidence, and ‘can-do' attitude that you get at girls schools”.

Magdalen College School in Oxford is the solitary boys school in the top ten for A levels this year. It missed the top position by a whisker. Although it also achieved 99.6 per cent of A and B grades at A level, it scored a slightly lower proportion of A grades (81.5 per cent, against City of London's 86.8 per cent).

It is the first time in at least five years that a boys school has come in the top of the table for A and B grades.

Tim Hands, the Master, attributes the school's success to its broad curriculum, which goes well beyond the A-level syllabus and includes two hours of community service a week during lesson time.

Next year, of the eight hours a week allocated to each sixth-form subject, one will not deal with the syllabus.

“I'll be teaching English to the lower sixth and will give them an introduction to literature from the 16th century to modern times. It's all a bit old fashioned, but it works,” he said.

Westminster School in London, a boys school that takes girls in for A-level study, tops the table of co-educational sixth forms, with 98.8 per cent of all grades at A and B.

It is closely followed by Brighton College, which is coeducational at all ages. Richard Cairns, the headmaster, says his pupils have benefited from a decision two years ago to teach less but do so at times that best suit teenagers. The school used to have lessons virtually non-stop from 9.15am until 5.30pm. Now there are activity slots from 11am to 1pm twice a week and from 4pm to 5.30pm every day.

“We actually cut five lessons off the week and gave that time to enrichment activities - sport, public speaking and so forth. Because the pupils had an outlet for their energies beyond the classroom, they were actually much more focused in the classroom,” Mr Cairns said.

The league tables are based on figures released today by the Independent Schools Council (ISC), which show that more than half of entries from ISC pupils were awarded grade A.

More than three quarters (76.9 per cent) of entries were graded A or B, compared with a national average of 50.8 per cent.

The improvement in performance this year is broadly in line with A-level improvements across all education sectors.

The raw results shown in these league tables provide only part of the picture of each school's overall achievements. But many heads believe that they provide vital transparency. Parents can learn a lot more about schools from visits, Ofsted reports and school profiles or prospectuses.

This year about 50 independent schools have boycotted the league tables. As they include leading boys schools such as Eton, St Paul's and Winchester College, the effect has been to bolster the relative standing of girls schools in the rankings.

Head teachers of the boycotting schools claim that league tables penalise schools that take weaker pupils, and encourage teachers to push students into softer subjects, in which they are likely to gain a higher grade.

The diversity of exams now taken at different establishments has also made it difficult to compare schools, others claim. A growing number favour the international baccalaureate, while others offer qualifications such as Pre-Us and I-GCSEs.

At Cheltenham Ladies College Mrs Tuck, who is also president of the Girls' Schools Association (GSA), said that schools' attitudes to league tables were often related to how high they were ranked. Although some high-profile boys schools had withheld their results, she and other members of the GSA did not want to follow their lead.

“We felt we wanted to carry on demonstrating the success of the independent sector. With other schools missing it may be harder to prove the strength of the sector.”

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