It's really frustrating. My Sims have missed classes because of different reasons.
My Sim went to class way too early and obviously got bored of waiting and decided to go get a glass of water from the dorm (this water obsession was supposed to be fixed already). I couldn't cancel the action because she was already going up the stairs (in Foxbury). By the time she got up the stairs she was already about 45 minutes late and when I made her go to class manually she decided to pull out her bike and ride, but because she was at the top of the stairs, she went "oops, what was I thinking, I can't ride down the stairs, I need to get off my bike" and stepped down the bike and put it back to her inventory again. When all these shenanigans were over and she had finally walked down the stairs she had already missed the whole class.
Another thing that happened was when my other Sim missed his final exam, because he didn't get the automatic "go to class" action for some reason. I had to send him near the building manually, but he was already late and because I didn't know which of the two buildings was the right one, of course it was the wrong one and he didn't make it to the other building before the final exam was over. I rage quit the game without saving. :/