I wish they would at least maintain their current grade, not average it down. Also lots of NPCs seem to be on probation for some reason. My sim made friends with her roommate so I wanted to add her to my household. She was attending classes with my sim for two/three terms. Yet now I've added her to my household, she has 6 credits but is on probation and has an F. I'm fine with her having credits as I attended with her, but she shouldn't have an F or be on probation. Can't find a way to cheat this to fix it either :(.
I think the issue with freezing progress would be what if you left a household at the start of a term and came back on the last day - if no progress was made, would they instantly fail all of their classes if you only played them at the very start and at the end?
I think for me, what I want to do is be able to enroll in a 0-class term. So when I move away, I can keep sims enrolled and in dorms, not have to reapply for anything, but have a term with nothing so it doesn't affect their grade. I might look into if there is a mod for this/if I can create one as this would solve my problems.