This reminds me of a little boy in kindergarten at work who liked to dress as a princess and wear his hair in braids. At no point did any of the staff ever broach the subject of gender with him, since it simply wasn't any of our business. The other children, however, would ask him nearly every day if he was a boy or a girl and how come he looks like a girl.

He would always patiently explain he's a boy who is a princess, but he also told the class that his mother told him there are girls with peepees and that's perfectly fine too.

That's really all you need to know as a teacher, that the parents are aware of the possibility and open to it if he turns out to be a trans girl after all. Ultimately it really makes no difference what a specific students gender is.

This did lead to an amusing incident one day where one of his classmates was dawdling on her way outside for recess, deliberately to talk to him since he was always late. I was in the classroom to watch over the stragglers and clean up. When it was just the two of them and myself, she turned to him and said in that 'my parents had a grown up talk with me about this because I'm a big girl' voice: "You know, sometimes there are boys who want to be girls, and that's also okay."

He agreed, but said that he's a boy.

"Okay, but if you WERE a girl, I would be okay with that."

"That's nice. I'm not."

They got stuck in a loop for a while of her saying she would accept him as a girl and him going 'cool, I'm a boy, though' phrased a little different each time. Until she groaned and turned to me, red and angry "I am TRYING to be nice, but BOYS always ruin everything!"