A high pass rate only tells you that the school is good at judging pupils’ chances of success before entering them for exams. (Put one successful pupil in for one exam and, bingo! – 100% pass rate.) The school’s pass rate won’t tell you how many A-levels your child will pass. If your child is enthusiastic but of marginal academic ability, you may prefer a school that will simply let them have a go. To find out what really goes on, find out the number and grades of GCSEs and A-levels passed by real pupils.
Pupil-based exam information is depressingly hard to find on many schools’ websites, where they are keener on giving you the number of passes or A grades by subject. This may be revealing but has to be judged against the national results in the subject (eg most pupils taking GCSE Greek get grade A but only reasonably bright children take Greek; almost all pupils are required take GCSE English, so the spread of grades is greater).