What is important to you?
Ethos - do you want pushy, laid back, liberal, strict (affects behaviour, homework, uniform etc)
Pastoral - does your child have any particular pastoral needs, are they likely to be target for bullying, suffer from anxiety
Extra curricular -do you care about any particular type of extra activity
Academic - does your child have SEN, or is a high flyer.
Curriculum - do you want any particular subjects to be offered. Do you care whether they can do 2 MFL at GCSE, or who gets to do triple science. do you care if they are forced to do PE or a tech at GCSE
Having worked out what is important to you, you know better what to look for.
e.g. For my DDs, pastoral, caring, good with mid/low ability, not too pushy, were the top things. I didn't care whether they pushed high achievers, or had 17 sports on offer.