I think it's our school district rather than necessarily (or only) the camp. Ann Arbor is very liberal, especially compared to the rest of the state, and the schools have policies in place about supporting asserted gender identities. My daughter's tiny middle school has a Gay Straight Alliance group (she's been part of that since it formed during the first year there was a middle school).

It is possible that Interlochen has policies, too. The primary thing it is is a boarding school for the performing arts at the high school level. The camp is just for keeping the place occupied during the summer. It draws groups like our high school music programs because it has top notch facilities for practice/performance.