I play rotationally, and when I age up each University class, I take a bunch of their townie classmates, throw them into an empty lot, enroll them all in University, and then evict them back out of the lot. That way I have options for roommates, but also they see their peers around campus as they go through. Interestingly, when I leave these NPCs alone, some of them end up graduating eventually while others fail out. It seems pretty random to me and I don't know if skills or traits influence it.