I assume it's a single gender high school and a mixed sixth form...
Thing is, with School A, the teachers know you, the school knows you (so you might find it easier to get in, or that was how mine worked). If you underperform on results day, they could say "look, we know you, we know you can do better, so we're going to give you some leeway" plus the teachers already knowing you means they already know what you need to perform best.
BUT
if you genuinely believe School B is better, with superior resources and teachers and results, that's not something to ignore. Like, at all. If you hadn't said that I'd have said A, no contest, but the thing is, you're likely to need those resources and teachers and results.
You need to take a step back and look at it objectively, and think that if you had no personal attachments to A already, what would you choose?