That's what Ryan Murphy,  Glee's creator, has decided..

Creator Ryan Murphy told Ryan Seacrest that some key characters (Rachel, Finn, Quinn, Puck) will be graduating  at the end of the third season.

“That is true. I don’t think of it in terms of eliminating or replacing. Because I think the thing about this cast is people love them and they are incredibly talented. They’ve left sort of an indelible mark,” he shared. “The thing that I wanted to do and the cast wanted to do, we didn’t want to have a show where they were in high school for 8 years. We really wanted it to be true to that experience. We thought it would be really cool if we were true to the timeline.”

While it may be the best thing for the series, (who wants college age students in high school?), it's sad to know it will come to an end. Enjoy them while you can!

source: justjaredjr