At the end of last season, the only person who seemed to be leaving was Brittany, for MIT, and they made a vague mention of her starting early, so it must not have been the end of the school year -- although they didn't really seem to care if we thought it was, too.
There was a big graduation at the end of season 3, and another one now, so in some twisted roundabout way, I guess a year has passed over 2 seasons. Or it hasn't.
I found an article where a reviewer complained about all this about a year ago: Glee: Finale points to weird timeline. What do you want to see next? | PopWatch | EW.com