The stakes are higher as the franchise moves to the big screen, but the cast members still act like kids. YOU MIGHT expect the set of “ High School Musical 3: Senior Year” :-senior-year to be a bit like summer camp, with a bunch of young actors running around in shorts and T-shirts. And to a certain extent, it is. During a recent visit to a night-shoot set in a junkyard here, the preternaturally familiar cast members are surrounded by the colorful rusted-out carcasses of cinematically groomed classic cars and are killing time between shots doing what any group of high school or college kids might do between classes. ...