An In-School friend is a friend that can only be talked to in school.
You do not hang out or associate with In-School friends off the school campus.
In-School friends usually are in one or more of your own classes, and they usually sit next or near to you.
Conversations with In-School friends are based meerely off the class you're in, how the other person thought the quiz/test was, if they understood what the teacher was talking about, and if they can help you on a problem.
It is very unlikely that you would have a conversation with an In-School friend on anything else except something related to school.
1. Conversation: "How did you think you did on that test? I think I failed it!"
2. It's very awkward running into In-School friends in public places, are you supposed to say "Hi" to them?