IMO, the teacher erred in that, the way it's presented, it's not about the he / she thing, but about the name of the kid that is "approved" by the parent.
Let's say the kid's "parent approved" name was "Airplane". There is a distinction there between that and referring to the kid "as an airplane".
He made it about religion instead of the facts of the situation. If his position was that he, ala Jordon Peterson, wouldn't call a "he" a "she", I think he'd be on sturdier ground, but still "processed out".
Yup, I don't know of a single religion that has rules about what names people can be called. Pronouns, sure, that could have a religious component. There is also such a thinking as knowing to pick your battles; this teacher has poor judgement in this regard.
See, I think that's the wrong view on that. It's not saying they aren't human, just that they are not what they think they are. I'm not an airplane because I call myself an airplane and you would be right to call me on that.