It's been a while since I played Uni, and when I do I only play one term per rotation. I do play with aging off, and I'm pretty sure NPC students have their own and individual schedule, meaning they have certain term points already, and when their individual score is full, they will graduate. Some students will leave, but I think the majority will move on to the next term. Next trm there might also be new students to replace those who graduated.
Added: The only way to control the NPCs would be to make the dorm a residential lot that is played like a dorm - this is how I do it. You can let rooms to NPCs, and the c harm of roommates bringing food, repairing stuff, cleaning etc will still add the dorm flair to your controlled dorm.