Have you managed to survive and thrive while actually engaging in school for the full hours every school day?
I've played a couple game weeks as a student. The first thing I did (outside of classes) is to get access to Tony (the drug dealer). You could do this by building up a relationship with the addicts in your class (needs 50+ relation) or by playing against Tony in the strip club (requires taking out a loan at the bank first).
My daily schedule as a student is work out in the mornings, go to class, football in the evening, sell steroids at the fitness center*, learn programming on the computer, and finally girls. After a while of nothing really changing, I checked the code briefly and couldn't find any reason to attend university. The diploma does absolutely nothing.
As for your grades, it depends on the percentage of time you pay attention (not distracted at 70+ horniness) in class. It doesn't depend on total knowledge level (ie not dependent on total number of classes attended) at all. Studying in the library or outside of class is only for making up for times that you show up to class distracted.
In regards to football, your results are almost entirely dependent on the number of times you attend training. Scoring a goal is 0.1% or 1% chance depending on how many training sessions you've attended. Your pay is dependent on the number of training sessions attended (and whether or not you scored a goal). Your stats have no influence at all.
* Selling steroids will net you 50 per dose if demand (randomly chosen from 1 to 5) is higher than supply (the amount of steroids you have on you).