Ideas
Just ran into this myself. Had my sim do the usual routine of completing all homework, taking notes in class, finishing all "final projects" till they were outstanding quality, and still got D's and F's. The sim did everything I done before to get A+s. Only thing different this time around, was I was in an apartment in San Myshuno. I was waiting to send the other sim while the first one finished. So, I will be seeing if it still happens from doing it on a residential lot as well. The sim it happened to stayed home instead of going on campus if that helps as well. This never happened before whenever I sent a sim to Uni and stayed home. Will try to remember to give an update if the same thing happens while doing it from my residential lot, but I would maybe look into the apartment stuff being the cause, since that's the only thing I personally did different since last time.
Do report back if it works, but my sim is in university housing and it's the same problem.