When you go to uni, you'll go with very few (if any) people you know. So school A would give you "practice" of making friends, as it's probably a long time since you've been in an environment where you know no one.
You've already got something in common with people at that school, and more so with people doing the same subjects. So you should find people you like there, and can still keep in touch with your friends out of school.