Id depends on situation. If a school break then: See you in xx class. I have to go now - Is more going away long term. ( I have to go now, though i see you (time) . Or just *Catch you later/ see you later) For number 2 i would use: *During class listen/pay attention to the teacher*

Either ‘in’ or ‘at’ is fine in the first sentence. In the second one ’in’ is better. I think the reason is that ‘listening to the teacher carefully’ is during the class [in the class], while seeing your friend is ‘peripheral’ - not part of the class itself.