They might have started the issue, but doesnt mean we need to play tit for tat on the other side. Who honestly gives a fuck if the teacher hasnt acknowledged a kids sex in a letter? This is so petty. The teacher is not going to make any impact on anything.
The trans/NB issues are only one part of it, though. It's a basic common decency, regardless of that. Suppose your name was Miss Nesmith and the teacher sent you an email starting 'Dear Mrs Smith' - on the grounds that, if you're just going for the lowest common denominator, more school mums will be 'Mrs' and far more people are called Smith than Nesmith - would you be happy with that, even though it most certainly isn't your name?
There's a bit of a parallel, I think, with Wales and the Welsh language. Whenever first-language Welsh-speakers living in their own country object to signs, communications and the like being only in English, they've historically been slapped down with "Oh, don't you be so petty - you speak English, don't you? What a silly thing to complain about!"
Nobody whatsoever is objecting to monoglot/primary English speakers having things in their own language, when a preference is registered (for things like official letters) and for signs and notices in general being bilingual - BUT if a Welsh-speaker has clearly indicated their own preferred language (an official one in their own country) and it's just ignored by people who also speak the language fluently, but decide to overrule them with "Oh, English is 'neutral', so that'll just do for everybody, regardless" - how exactly do you think that would make you feel as an individual?