I think this is more about treating a transboy’s feelings and safety in regard to situations where they might be naked or partially naked around other boys in the same way they are supposed to treat girls’ feelings and safety around boys.
Sorry, I’m not clear I’ve understood the point you are making here. Your use of “other boys” creates the impression you think “transboys” are male pupils. But they’re not.
When you say “transboy” are you using that term to refer to a female pupil who asserts a masculine gender identity? (Which is what Stonewall, Mermaid etc mean when they say transboy.) Or are you using “transboy” to refer to a male pupil who asserts a feminine gender identity?
You compare the “transboy’s feelings” to how a girl would feel about being in a state of undress in the male changing room. Which would be a relevant comparison if you’re using “transboy” in the same way as Stonewall would - ie to describe a female pupil who self describes as a member of the opposite sex or as having a masculine gender identity.
But that’s at odds with your use of “other boys”, which I took as meaning that you think a “transboy” is a male pupil. It shouldn’t be seen as “inappropriate” for a male pupil to use the male changing room just because he is gender non-confirming.
Comparing that male pupil’s situation with the situation of a female pupil forced to share a changing room with the opposite sex when in a state of undress is not a relevant comparison.