Tips to pass University Classes with A+

I'm completely confused how the grading system works. Or, maybe there is a bug in the game that causes failing grades?

I have a Sim who completes about 75% of his homework, goes to every class, submits excellent quality term paper, good quality presentations, attends class every time on normal setting, gets the pop-up that he squeezed out every bit of learning from the class, takes the final exam, and he FAILS the classes with an F!?!? Ok now I don't expect him to earn an A. But I also don't expect a failing grade, either. 

I've read all kinds of hypothesized reasons for failing grades on the forums. They don't make sense to me and I think there is another cause. Going to the library to study, or setting the attendance to active listening or take notes, shouldn't be the reason a Sim FAILS a class. Not being a level 10 in a related skill shouldn't be a reason to FAIL a class. If a term paper or presentation is submitted before the final day of term, that shouldn't be a reason to FAIL a class. For every post that claims these are reasons, there is another post who says they don't matter.

This is why I'm convinced there is a bug. If not a bug, I am thinking possibly that there might be a minimum skill level required for a class, and that minimum skill level rises for every subsequent class. That might explain why my Sim started with As, and then the last term of 4 classes he got C, D, F, and F. 

Or else, the SimGurus could explain how the grading system actually works, if there is some other unknown criteria for passing a class. I'm tired of reading posts where everyone is just guessing at all the reasons for Failing grades.