Kathleen ConnerFinancial ConsultantCalifornia State Thespians
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You are correct in the idea of casting 9th graders in children's roles. Many of them really do look considerably younger than their 11th and 12th grade peers. You are also correct that you can often cast girls in young boy roles. It's pretty much required in a show like Peter Pan for example. But yes, in a show like The Miracle Worker, a 9th grade Helen Keller and a 12th grade Annie Sullivan works great. I recently cast a small for his age 9th grade boy as Les Jacobs in Newsies with equal success.
I have a small school so I have to carefully pick shows that suit my kid's strengths each year. It sounds like you already have your show picked. If I had a lot of strong freshmen and they all looked like little kids next to the rest of my students, but they were still my strongest performers, I might pick a show where kid characters were the leads, like Matilda or School of Rock.
I think casting smaller girls as the boy and the girl should work great for you. Especially in a show like Chitty Chitty Bang Bang (or Mary Poppins). It's pretty easy for girls to play the proper young British boys in those shows and it actually fits the styles of the piece.