Julia, are you talking just in your high school musical for now or professionally?
People have covered most of these wonderfully, but obviously there's a lot more to do professionally. Still, there's fun things to do in high school. OF COURSE my first suggestion is going to be to play in the pit if you play an instrument.
If you are willing to work hard, you could stage manage. It's a lot of work and you're responsible for the running of the show, and though it's stressful, I had a lot of fun doing that.
You could also be production assistant, which is slightly less work and you can go more into the professional behind-the-scenes stuff. You assist the producers in advertising and making tickets, you assist the stage manager and/or director at rehearsals, and you get to be everyone's bitch. Fun stuff.
Props mistress, as Dre mentioned, is also a fun possiblity.
Crew in general of course. In high school, they're always looking for help there. There's lighting, sound, constructing sets, painting/artistically creating sets, etc.
I think that's most of your high school options.
Of course professionally, there are so many things to do--production, literary management (deciding what shows to put on), development (raising money), publicity, marketing/advertising, general/company management, talent agent, casting director, etc. Try to do some research, there should be good information online. I'm a member of the theater intern group here, and we get monthly seminars from professionals in the different jobs I mentioned above. Find out if there's something like that around where you live.
Good luck!
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli
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