Thing is, mammals (including humans) can't change sex.
Moreover, there is no such thing as gender identity (see, e.g. Alex Byrne (Philosophy Professor, MIT), 'What Is Gender Identity? - The elusive true gender self': "... If there is some kind of “gender identity” that is universal in humans, and which causes dysphoria when mismatched with sex, it remains elusive. No one has yet found a way of detecting its presence ...".)
Given which, it would appear there are no trans children at school or anywhere else, not in truth.
Of course there are some who say they have changed sex, and some who say they have a gender identity. (Children make mistakes ... and, even, I have known one or two who told fibs, believe it or not.) How should we treat such children? Possibly in the same way we treat those who say they are cats and so on? - With sympathy (always!), but also with appropriate humour and common sense. Hmm?
How do we treat thin children who think they are fat and so want to avoid food? Perhaps there's a moral in that for how we should treat girl children who think they are boys and so want to avoid puberty: try to help them, best we can. (But not, 'help them avoid food' or 'help them avoid puberty', of course ... That would be awful, wouldn't it?)
(Of course waters might be muddied by the legal fiction of Gender Recognition Certificates. But that determinedly does not apply to children. So leave that be for now. For children, matters are clear.)
As far as children are concerned, it's well past time to consign all the trans rubbish to where it belongs: in the bin.
Once again, there are no trans children, in or out of school. Thinking you are doesn't make it so. (Which, again, is diametrically opposed to being gay: thinking you are attracted to your own sex just is being so attracted, so thinking you are gay does make it so. Parse this how you like, 'LGB' just doesn't match with 'T'. Not at all. Not in the least.)
Tldr: If a trans child is one who has changed sex or one whose gender identity is such-and-such, (doesn't match, blah, whatever ...), then there are no trans children. None.