'i catch you up in class time' would be incorrect. The phrase usually used is "I'll catch up to you in class"which means I'll see you in class.

It doesn't sounds quite right. Do you mean to say" I'll catch up to you in class" Say you and your friend are on you way to class. All of a sudden you forgot something. So you decide you want to return to get the object you forgot. Then you tell your friend. "I have to go back to get something." I'll catch up to you in class"