If a sim gets an F in a class do they have to take it over?

I had a Sim who was taking Biology as his major, since he was going into the medical field. Getting him to do all of his work got increasingly difficult to achieve. Then he managed to be late to class more than a few times. Or I forgot to make sure he was taking notes while in class. He was living in the same dorm as his three siblings, at that time. It may have been my first attempt to guide that many Sims through Uni. All I know is that they each had three classes apiece. The long and the short of it is, Joey failed one of his critical classes. (At least I saw it as critical.) But, he failed. Since I wanted him to graduate at the same time with his siblings, I had him take four classes, to 'catch up'. And, yes, the very class he failed was one he could take over. I don't know about it being random. I have taken Sims through Uni (generally multiple Sims at a time) at least three times at this point.