Little theater groups, drama clubs, whatever is available. Every minute spent on stage or behind the scenes working on a production pays off at some point. Look to see if they've worked on musicals rather than straight plays. The singing, the dancing, the dialogue where you have to land a point in just a couple of lines. The experience of performing before an audience.
Musicals are a good context for a kid's natural exuberance.
Even if it's, God forbid, the annual High School production of Oklahoma , it's worth it. The National's doing a revival in London. It's fine for what it is. All I'm saying is those High Schools that even attempt musicals pick from the same dozen or so shows year after year. True, Into the Woods is now a staple and you even see the occasional West Side Story .