That's interesting!
I agree that skill level might be essential, as the responsible trait. I assume also the "happy toddler" trait might help, making them build skills faster.
My guess is that many simmers send their student directly from CAS. Even if cheating skills, I think there might be hidden factors that only a sim who lived in game will build. That CAS sims has a bigger chance of failing than sims who grew up in game.
I do play most of my students, but sometimes I left them on their own and they did not get A+ but they did pretty well. That was not for the entire term though, more like the final couple days.