I never played TS3 Uni, but yes, you can have many active sims in the same Uni household, being dorm or residential. BUT - you should plan for playing them through their term before switching households, or they will perform randomly/average. Their progress for current term won't be paused while you are playing other houses. It's not a problem though, you just need to play them for 5-7 days in one go.
You can add new students to the existing household at any time, but if you plan playing the students for only 1 term at the time, you should avoid adding a new student when the others are mid-term. Make sure they all start their term on the same day, but they can be on different terms, no problem.
Also, when a term is completed, there will immediately be a popup urging you to reenroll for the next term. Playing on rotation, you will say no to that, because you want your student to take a break while you play other houses. If staying in student housing, when a students choose not to reenroll right away, the student is told to pack bags and move out, as you can only stay in say a dorm while enrolled. Unless you want your student to move back home, you could prepare a residential lot (any world would work) for temporary living. If staying in a residential lot (can be organized as a dorm) you won't need to move out while taking those breaks.
Hope this made sense :)
