I think that the passing grade has a lot to do with skill levels. It seems like there are hidden skill level requirements in order to succeed in each class, and the more you do homework and study for those classes, the more those skills go up. But if you bring in a Sim who started with a very low level in the needed skill, the work they do might not boost it enough to get a decent grade, so they fail, even if they're doing all of the projects and homework.
I was having some random problems like this as well and someone else mentioned this caveat. I haven't really brought any new Sims to Uni in a long while because I play a bazillion households and don't want to be stuck in that gameplay, but I do think there's something to this. The devs did warn that Uni was designed to be challenging so it was most likely intended.
Try making note of which skills are increased by which classes and maybe try to do a little extra boosting for those skills and see if that helps.
