You have three roommates and one of them is awful. She's a pretty shit roommate, as far as roommates go, generally making everyone else annoyed with loud music she won't turn down, having parties and alcohol on a dry campus, and saying really mean things about you and your other roommates on facebook. You've tried numerous times to fix the problems (from mediation to trying to get her kicked out of the apartment...) to no avail. She posts something about blowing up the cars of everyone she doesn't like.
Your campus takes things like this very, very seriously. Precedent says, she's likely to get expelled. Do you turn her in?
Does it make a difference if all four of you, including the bad roommate, are seniors and less than three months from graduation?