Well, let me explain about university and rotational play.
First of all, it is important to know the difference between played and unplayed households. Unplayed households are those you haven't played with. In the map screen they're represented with gray (if not in my households) and light green (if in my households). They can change their jobs, don't keep much of the relationships with other unplayed sims (unless related) etc. Played households are households you have played with at least once. They are represented with green plumbob icon.
Now then, if a sim is a student, their term goes on no matter if you are there or not. They will finish it on "average grade" and what happens next depends on whether they're played or not. Unplayed sims will reenroll on their own and will do so until they've graduated. Played sims will wait for the player to return so the player can re-enroll them.
To keep my sims in university housing and not have their grades suffer, I usually do this.
I play with them the whole term. Once it ends the game asks me to reenroll them. I do that and choose them one class. After that I rotate away and don't return before a week passes. Once I return, I reenroll them normally. I usually want to go through three terms with students, so the amount of classes they take is 3, then 1, then 4, then 1, and finally 3 (or 4 if I want to go over).