koreelovee12 I am currently playing an 8-sim household in uni (all born in game with lots of reward traits and bonusses). I have had no problem with grades so far. My sims do the presentations, term papers and homework. They go to class and take notes while there. The don't do anything else. And they get A + mostly. Worst grade was a B that was my fault as I missed that he didn't go to class one day. So, the issue must be in your game setup. So, if you are using mods, and especially any mods that affect anything doing with university or even high school, I suggest you take a hard look at them and make sure they are working correctly. Try playing without them to see if that makes any difference. It also probably wouldn't hurt to do a game repair too.
Hope this helps.