Misgendering someone is an insult like calling someone names, or bad mouthing someone. There is no fundamental right to insult other people, even when you rationalize it with beliefs that happen to be protected.

For example,

I might believe whatever I want about my neighbor, eg that he is a fascist cunt, and I am allowed even to say so in private.

But saying it to his face is like a breach of the rule of law, as is saying so to others. I might think he should be lynched daily, but saying so might well be a crime.

You might even say that ignoring him in the elevator when it is customary to greet your neighbor, although not illegal, it is considered just rude by society standards.

So at the very least we have a teacher being systematically rude to his students for religious reasons (or “Gender critical”, all the same), thus making him a dick. See my recent comment on Maya Forstater for some quite similar case, only this asshole is aggravated because he is in a position to scar kids.

Even if your belief is protected like religion, or you push it to be (TERFism), you have no right to violate another person’s dignity because of your beliefs.

Bigots are bitter about it, and that is why they want to destroy the constitution and the rule of law to have their way. By extension they are against some basic principles like freedom of religion (of others).

Plus, there is research that shows that respecting pronouns is a mental health protector for trans and non-binary teenagers, so this make the teacher a perpetrator of demonstrably abusive behavior towards his students. For these reasons I believe he was quite rightly discontinued, and I would believe the same if he were outright terminated.