I agree, Rosie. As a union rep I saw cases where that sort of undermining of staff was treated as evidence of bullying and taken very seriously.
If we are being hypothetical, what might have worked better could have been for the teacher to be given the chance to explain firstly whether she had even known about the non-binary child, and if she had known, but had chosen not to choose a different word from 'girls' why she did so. The children need to learn that people disagree, and just because they hold a point of view doesn't entitle them to force everyone else to comply. It could have been done politely and reasonably, if each 'side' had been willing to listen to the other.
Also hypothetically, what about a girl in the class who doesn't believe in 'gender'? In Glorianny's fictional world where the school ethos dictates what can be said, would she be allowed to express an opinion? Would she dare to speak out even if it were allowed, given that teachers were forced into humiliating apologies?
I'm sure we could all invent scenarios that will lead nowhere, as they are not real, and therefore cannot be proven or disproved.