Your question is not enough clear to me . You can discuss on any topic in the class or even outside the class. The discussion depends on the situation you pass or relate to. People discuss formally or informally. You can term it the way you want.

usually if you have a discussion in class with another class mate im assuming. it is to discuss the class topic, to understand another persons point of view to get a broader perspective and to learn more about the topic. If your just casually talking to a friend, you are talking because you want to talk to them and have the freedom to talk about other things if you choose. In a class its mostly just for learning and discussing a topic.

There is no difference. You both talk about crap..... oh you were being serious?

Okay, a college class discussion is where you talk about the subject you're learning in a formal manner and maybe flip tables on which ***** stole your #2 pencil.

A casual conversation with a friend is where you can be informal and still flip tables on who ate the last oreo.