I don't see the harm in schools allowing children to be called by the name and gender they identify with at that time?
Dr Cass saw the harm, and so do Transgender Trend. The links are upthread.
And it's not about "allowing" anything - it's about insisting that every teacher and every other child uses the new name etc, and deliberately hiding it from the parents. Meanwhile the parents have no chance to understand, to learn, to adapt, to even know what their child is thinking and doing.
Defaulting to secrecy over pronouns reinforces the message that children cannot trust their parents about gender.
It doesn't necessarily mean they will transition fully, but if they did would it not be better for the children to know their parent loves them and supports them no matter what?
Parents don't always support our children by giving them what they want right away.
The kids that ask school to keep it secret must feel like their parents wouldn't accept them as who they are, and that is dangerous
"Accept them as they are" is begging rather a big question - what are they? A loving parent can accept that their child is questioning their gender without agreeing that they need social transition right now.