At least where I went, it was perfectly possible as long as they were compatible roommates. We had stuff to fill out like "are you a night person or a morning person?" and "do you smoke?" and other questions like that.
It doesn't make sense for Sarah and Joyce to room together not because of their age but because they are utterly incompatible as roomies. I can't imagine my old college doing ANY of these roommate assignments.
Willis intentionally put everyone where he did in order to create as much fake conflict as he could. But in reality, the roomie system is normally much more accommodating. And with it being a month into the school year, at least a third of them would have left the floor entirely.
A lot of freshmen quit after the first week of classes, after the first month, and at the end of the semester.
Plus, a lot of people don't like their roommates or they met people they liked more, so they move in with people they don't hate. This floor could have been fixed, at earliest, two weeks ago, in-universe.