You typically can't get an "internship" without a connection to some sort of a university program, for credit. I believe this has to do with union contract rules. If a theater is big enough to do internships it's probably a
union house and then the union rules kick in. Fortunatley, theaters that aren't big enough to run internship programs are often looking for volunteers. The most productive thing for a high school student to do would be to
call around to your local theaters and just ask. Tell them you are a hard working student and you want to volunteer. My targets would be larger theaters that run summer drama camp programs (ask if they need help lighting the show). Or smaller community theaters. If you find the right community theater you can really learn a lot and
build some great connections.