A lot will depend on your child's interests and what subjects they're best at.
If they excel at Maths, ask whether the school does Further Maths at GCSE and A level?, Whether they enter a team for inter-school Maths competitions, etc.
If they're into football, check that football is actually an option during games lessons (this caught out my son, he "assumed" his school would do football, but they had both winter terms of only Rugby!).
Do they offer separate sciences or just the 2 or 3 combined science option?
What languages do they offer at GCSE level? Do they do classics (i.e. Latin or Greek).
If they're into drama, ask about the school theatre, what kind of "performances" do they do? How many per year?
Schools vary enormously, so you need to find which is the best "match" to your child's expectations and requirements.
Perhaps most importantly, ask existing pupils about pastoral care, bullying etc - all schools have a very similar copy & paste "policy", but you need to know from pupils about the reality of the what the school actually does, not what they say they do!