I had this problem too. My Sim is doing Biology at Foxbury and her classes were scheduled with five minutes between, but were in different buildings. It would take 40 minutes or more for her to get out of class, and then walk across to the other building. Sometimes she missed the next class altogether.
I made sure she did all her homework and extra study and she got an F.
Either the timings need to be better, or the classes further apart. The walk from one building to the other should take a max of 10 minutes, not over 40! Or like you say, the classes need to be in the same building when they're close together.
