If you say, "friends in school," that means your friends are students. If you say, "friends at school," that means they are people you know because you go to school together.

If you say, "friends in school," that means your friends are students. If you say, "friends at school," that means they are people you know because you go to school together. (Update: I meant to leave this as an answer, not a comment. Sorry for the duplicate.)