Oh wow, so now the GLP are lined up against you as well as the school😐
They appear not to be very good at winning, so in fact their presence may tip the balance in your favour!
As PPs have said, allowing boys to be in girls' facilities is far removed from 'protecting all children', but I guess boys wouldn't be very pleased at having a girl in their changing room - boys are entitled to privacy too.
This is assuming that a trans-identifying girl in a boy's changing room would not be harmed, that the boys would instead be embarrassed by her presence; but supposing things took a nasty turn and they did something to the isolated girl?
The school would have difficult questions to answer about the wisdom of their laissez-faire attitude to changing rooms and toilets.