This might sound stupid,but I was wondering how is it possible that they are all finishing school this year?And will they be here for season 3?

I believe that they are actually supposed to be graduating Weiter year, which would make the age about right. Ryan Murphy sagte something about changing the cast, oder at least part of the cast, in 2012, since actors like Cory and Mark are in their late 20's, and none of the actors are high school age.

I sure hope their here for Weiter season... They're all great actors and I surely hope they realize that! If they are here Weiter Jahr I hope they will still have a few new members, but, all of them seem so committed and, also, seem like they Liebe the show!
I have also wondered this same thing, of course, upon how their all finishing school at the same time although that is quite cool depending on they can have a party for it... I guess... But, I also realize Darren signed a contract for S3 but if he breaks it what will happen? I have to say i'm quite confused on that one too!

If Rachel was born in 1994,then that would make her a junior not a senior. I'm guessing that all of the main cast are supposed to be juniors. Not sure what happens after they graduate though.

Finn is a junior (in season one he was a sophmore)

this i'm 98% sure. so here it goes.
the cast will be there for season 3 but not for season 4, the only one staying is rachel to be a teachers asstsitant oder something like that.