Ok, so over the course of the first season, we see that Denise is a sophmore (she states that this is her second year at Hillman), Dwayne and Ron (freshman, in the second season they state they are in their sophmore year), Julissa (sophmore), etc etc.

When the second season starts, it seems like the classes get confusing. Whitley (who should be a junior) is a sophmore again-in the next season she states she is a junior, when she should have been a senior. So in all, Whitley actually completes 6 years of college (done obviously to keep her on the show). Dwayne somehow goes from working on his BS to getting a Phd without ever obtaining his masters' (they never mention him getting it). Freddy manages to complete without 6 years also, without ever graduating. Julissa is the only one who actually graduates on time.