I've only had a couple Sims graduate, but I can tell you what they did. Before they even went, I built up all the skills that seemed relevant to what I wanted them to do. I only signed up for 2-3 classes at a time, which makes it take longer, but made it easier to manage and left me a little free time to do fun things.
Once there, they always immediately did homework, papers, whatever else they could do. When any additional lectures or anything popped up, I'd do them as soon as I could. They didn't do anything else until those things were done. They always went to class as soon as the option to go was there. When I couldn't remember where a class was, I'd just take them to the area between buildings so they'd be closer at least when it was time to send them to class. And the biggest help was that I let myself cheat motives while there (I don't normally).
Both passed with A's.
