This is a genuine problem but there has to be some solution to it that doesn't involve non-trans girls having to be naked around people who appear to be and may in fact still grow up to be men (trans, doesn't like weirding people about about it).
That solution is, of course,
Compulsory group showers are weird and creepy relics that should be phased out
(normal).
ETA: Also, I know that this is a minor thing to get persnickety about, but the increasingly preferred practice is to say 'trans [woman/man/girl/boy]', with the space in between, rather than making it a compound word.