The problem is...
If jokes and memes are the only responses that win rounds, then soon that's all anyone will write, and the class will get stuck.
Stories will be pointless, writing skills won't improve, and you'll wonder if you wasted your time.
(This is not unique to students; Keith Johnston writes at length about the same dynamic with adults in improvised theatre. The issue is mainly about the desire to gain status and a fear of losing status.)
If that's your situation, use the next game to introduce Approval Mode.