2 classes isn't too bad. 3 it starts to get difficult. Believe it or not, my sim who is married with a toddler has it best. She has 3 classes but I feel like it's easiest for her to get stuff done lol. The husband does everything and she focuses on work and school lol.
It's almost impossible to have multiple people enrolled living separately. I have the Pleasants I'm playing and another sim and was going to try even more but then I went and saw Angela was on probation. I was hoping their grades would stay average. Like just B or even C. Just enough to not get thrown out. But nope. Basically if you want a Sim to get through uni you gotta stick with that sim.
It would probably work best to have sims you want to go to move into a dorm/house together I guess.