07/13/2011 AT 07:35 PM EDT
stars Lea Michele
, Chris Colfer and Cory Monteith will don caps and gowns at the end of the fall season and won't be returning for the fourth season.
Other characters also will be graduating from high school and their identities will be revealed in the Sept. 20 premiere, Glee
co-creator Ryan Murphy tells The Hollywood Reporter.
"We made that decision and I involved Chris and Lea and they thought that was a good idea," says Murphy. "They both trust the writing and trust me and felt that it would be great to have an open and closed experience for them to go out while they were on top."
Murphy acknowledges he hasn't discussed it with Monteith but, "He knows he was a sophomore when the show started."
The characters are leaving, Murphy says, for the integrity of the show.
"You can keep them on the show for six years and people will criticize you for not being realistic," he says, "or you can be really true to life and say when they started the show they were very clearly sophomores and they should graduate at the end of their senior year."