@Sindocat absolutely nailed it with "enroll on Friday." Having a whole weekend to get the heavy-duty study stuff out of the way course-wise before even starting the first class is huge.
I'd also recommend getting your college-bound sim up to Handiness 6 so they can put the faster-sleep upgrade onto their dorm bed or even their bed at home off-campus. If you're not going the "no need decay" rewards route, that upgrade will at least give your sim a few more hours in the day they don't have to sleep.
I've also found that great stories about a sim's post-collegiate life come from the people they meet at school. My sims almost never marry their high school sweethearts because some story inevitably presents itself at school.
Athletics or e-sports are a hideous drain on a sim's time; fulfilling those responsibilities will absolutely murder a sim's grades.
Going to class in a good mood is important, as is using the "Take Notes" workmode. There's a meter hidden behind the scenes that's basically the same as the meter that was very much visible to your sim in Sims 2 University and is similar to the Work Performance meter in any job.
The worst grade I've ever gotten with a sim was a C, but that was because she went into labor in the middle of a class and got counted as having skipped it. In general, I've never gotten anything short of an A when I've had my sim complete all their basic class responsibilities (go to class, Take Notes, do their homework, finish and fully refine their term papers and presentations.)
