Okay, I tried a few things.
First simply moving the mods folder did not work for my old save, nor did it fix anything for a brand new save (in retrospect, not sure why not? I did delete caches btw). So I thought it must not be mod related until...
I went ahead and renamed the entire sims 3 folder. Started a new save with completely fresh start and finally a class schedule appeared :)
I then copy pasted my old save into the new sims 3 folder and tested both saves together (with no changes in between, just quitting to menu and loading the other save). The new fresh start save worked, the old save did not.
And finally, I reintroduced my mods folder and tested the new save, and the problem reappeared a little differently. Instead of blank schedule, I couldn't even get the schedule to come up (pressing the button didn't do anything).
So my guess is, it is mod related, and it does corrupt old save games. I have yet to find a way to play with my mods and without this issue. One last thing I'll try is going back to a save from before university now and see what that does...
So in short, a complete break is the only way I was able to solve this. Good luck.
Ps. I have a long list of mods and I have little hope of figuring out which it is without testing. But seeing as there's so many people here, I can't help but wonder if it isn't something popular...