Katy Perry joins Darren Criss and the Dalton Academy Warblers to sing 'Teenage Dream'

In the off chance that “Teenage Dream” fatigue has not yet kicked in, last night at the Trevor Project Live benefit in Hollywood, Darren Criss and the Dalton Academy Warblers performed the mega-hit “Teenage Dream” with a very special surprise guest. About halfway through the performance, Katy Perry took the stage, sang some big vocal ad libs, and then joined the group for the rest of the song.  Click here.