If this show goes forward another two years, have most of the club die horrifically in a bus crash on the way to Nationals, plus kill off or finish off the New York side of things. Start mostly fresh. Remove the season one cast from regular status have have them come on occasion for cameos. Fill the glee club with the twelve faces, but focus on no more than five or six of them plus the teachers. Everyone else is background, or part of a rotating cast of extras as club students will enter and leave much like any other club in real life.
Joe is the Christian guy. He hasn't even had lines in the last couple episodes, has he? For the most part he's been occupying space in the back of the room, as I understand it. As an actor even he should be getting frustrated by being billed and paid as a secondary player but being given no more to do than the extras miming the instruments in the band, while this year's new cast have been getting all the plot. I'm guessing he's graduating this year too?
Glee's problem has IMO primarily been with too large of a cast, even in the first season. Not enough room for everyone, especially after the most popular characters get their literal solo.