There are a few factors that contribute greatly to your sims GPA, and thus, how difficult University may seem to be.
First - and foremost - Ensure that your sim arrives to class on time. This means that if you live in a dorm that you have to account for travel time for them to walk (or bike) to class. When you arrive to class late, you take a hit to your grade.
Second - Deciding how you spend your class time. As others have mentioned, just like when you go to work in the sims, when you go to class you can click on the seem and have them do a variety of things while in class. Sleeping in class will obviously have a negative effect. "Listening closely" boosts the skill gains they get from the class. And, "Taking Notes" increases their performance in the class resulting in better grades.
Third - Presentations, Term Papers, and Final Exams - These are important and can be done at any time during the term (aside from the Final Exam which is automatically taken during your last class). Make sure you do the Presentation/Term Paper and then your edit it to increase its value. It is a bit time consuming but otherwise not difficult to ensure you have a high quality presentation or paper.
Just by doing these three things, you can guarantee a decent grade in your classes. I just finished playing a student-athlete that was more focused on her athletic aspirations and social life than her school work, she rarely did homework and never studied but still graduated with a 3.3 GPA.
Homework - Doing the daily task of homework can be a great way to ensure your sim's grade, but it isn't necessary if all you are looking to do is pass. On the other hand, it is a must do if you are looking to grade with Honors.
Studying - This is never a required task at any point. You can get a 4.0 GPA without studying a single time. However, studying can be useful when you forget to "Take notes" during a class or if your sim arrives to class late. Think of studying as doing make up assignments. If something messes up and you sims goes to class 30 minutes late, then definitely spend some time studying (preferably at the library or another Study Spot).