This summer, two leading independent schools, King's Canterbury and Gordonstoun, boycotted league tables, arguing that they failed to give full, balanced pictures of pupils' progress.

Few schools are likely to follow suit. Love them or loathe them, league tables are now as important a part of the school calendar as speech day. As Jonathan Stewart, head of sixth form at George Abbot comprehensive, Guildford, confirms: "Whatever schools may say, most parents take notice of them. We have to as well."

One obvious effect of the tables is that they focus so much attention on academic success that other areas suffer. During last June's controversy over the introduction of A/S levels, David Hart of the National Association of Head Teachers, complained to the education minister that important extracurricular activities such as sport and drama were being forced out, as school summer terms became exam-based con-veyor belts.

The Perse in Cambridge is precisely the kind of Top 20 independent where the impact of league tables is most keenly felt. Facing a competitive local market, not only do such schools have to justify constantly their performance in the tables, they have to timetable carefully to preserve extracurricular activities.

"We do all we can to ensure league tables do not dictate what we offer," says Nigel Richardson, the head. "But in a place like Cambridge, these results really matter. We have a high proportion of sixth-form time devoted to sport and extracurricular activities. But we want to offer five A/S levels and four A2s (equivalent to old A levels) to most pupils."

Peter de Voil, head of Frensham Heights, Farnham, an independent strong in performance arts, sees things differently. "In some academic schools, league tables are taken so seriously education is reduced to the curriculum only," he says. "I know one head who claims academic standards have improved in his school because pupils are no longer allowed out on school trips."

With concern about schools "doctoring" their position by preventing weaker candidates from even sitting exams, de Voil is convinced league tables can affect school selection procedures. "Will a top-of-the-table school really take on aborderline candidate who might prove an academic liability?" he asks. "If we accept 'x', he might damage our position in the league tables.

"I've often heard school staff say this. Yet 'x' might be merely slightly dyslexic. I find such attitudes educationally frightening."

In the state sector, the same pressures do not apply to extracurricular sport, which is often optional in the sixth form. Yet, even if a comprehensive is totally non-selective, like George Abbot, league tables still have a big impact in boosting local popularity, as Jonathan Stewart admits.

"Because our results are good, we have benefited from league tables by increased demand," he says. "We're oversubscribed, and the tables offer a useful barometer of our local standing. But they have not affected our ethos, and we have not changed any of the curriculum to suit them."

League tables also affect the approach of teachers: the pressure is on to stick to neatly compartmentalised syllabuses, achieving good exam results at all costs. Anthony Seldon, well-known political biographer and head of Brighton College, believes they lead to a narrower education.

"League tables encourage teachers to teach the exam, not the subject. It's a game schools are forced to play or they lose out, as exams become more formulaic."