What I do is focus most of my attention on the sim(s) that I want to succeed. I rotate for maybe a few hours to my other enrolled sim, just long enough to complete some coursework. That sim still passed the term with a B+, even though I wasn't playing her at the time. But she flunked the remaining two terms. So either I didn't play her enough, or her low skills were at fault.
My guess is that the higher your sim's skills are, the less likely it is that they will do poorly when you rotate to another family. I'm going to test this theory a little more in my game.
