Though High School Musical 3: The Senior Years has yet to make its jazz-handsy way onto cinema screens, the Disney Channel has announced plans for a High School Musical 4.
There's no word on whether the original cast will be back, since most would presumably be off to college/dance school/re-hab after their senior year, but Variety says that some characters who are introduced in the third film may well be pushed to the fore in the fourth. We'd be surprised if Zac Efron came back, since he's making good in-roads into an actual adult acting career, particularly with Richard Linklater's Orson Welles and Me.
Also no word on whether this will be big or small screen, but early suggestions are that it will stay on TV.