I noticed that many people who went to Catholic school dislike Catholicism very much.?

Nuns and Priests as teachers aren't generally well liked and easy to get along with. People don't like to be forced to do much, and Nuns/Priests require strict obedience to not only their rules, but the rules of the church, and many teens attending Catholic schools can grow to resent the heavy strictness. Plus, the teaching, when you really get into it, doesn't make that much sense, and those that turn against it have "looked outside the box" as it where.