This sounds very hard OP.
I actually think for the sake of our children's mental health we have to be prepared to be honest and clear with them even when this upsets them or makes the relationship difficult in the short term.
This pandering framed as 'support' in going along with something absurd and untrue is dangerous: she is not a boy, and non binary is not a unique category, and you cannot control other's speech and you shouldn't aim to, or think your identity or mental health depend on others validating your identity, this type of 'support' undermines mental health and gives really unhealthy messages about reality and life expectations.
So I'd be saying to my DD: you have a female body, so you are a young women, we use she for women and I will not pretend you are a boy.
To the school I'd be saying: if you refer to my daughter as a boy in any way I will be seeking to take legal action against you.
Actually I wouldn't I'd remove her from the school.
I don't mean to suggest this is easy OP I'm sure they'll be some difficult times ahead, but I do think honesty based in reality is better for adolescents in the long run rather than this: just say the lie if they want you to, to be 'supportive' approach.
It's not support or kindness to lie and mislead.