I'm not sure meeting in the Great Hall is very likely. I think most students would work in the library or in the common room during their free time. They seem to be pretty much left alone by the teachers during their free time. Sorry if that messes your story up, but that's just the opinion I've drawn from what we've seen in the books.
If you need them to be in the Great Hall, it wouldn't be completely un-canon for them to start something like that, especially during the times when the school was in danger (Chamber of Secrets opened, Sirius Black on the loose, etc.). The teachers could have decided that the students shouldn't be left alone.
EDIT: I just thought of another situation.

I could see Umbridge, in her efforts to control the school, not letting the students be alone. She was afraid they were planning something against her, so she made them spend their free time under her nose. Just a thought.
Hope that helped!
~Chelsea