Flipping through premium cable channels offers hours of hormone-induced hip thrusts and shrieks of bliss, and Disney Channel’s hit franchise High School Musical blends right in with the steamy best of Skinemax. Clean teen idols like Zac Efron and the gang may act like goody-goodies, but their expressions of onstage pleasure (not to mention their porn-worthy names . . . Troy Bolton and Sharpay Evans?) make them almost indistinguishable from XXX stars. So go ahead and study these money shots closely and see if you can tell who’s trying out for a lead role at school and who’s boning up on their adult-ed performances.