I think if you leave Sims for a whole week, they'll fail the classes that they're taking because they won't do the tasks (homework and more importantly final exams for those classes) that need to be actively done and a semester only lasts 5 Sim days (plus the weekend when there are no classes). So you'd be putting them at risk of failing.
I don't know if there is an option to retake a class.
You might be able to get away with it if the Sims can only take electives in the week/semester when you're not playing them. No idea if that will be possible.