Yes, many, many schools took on policies from Stonewall, Mermaids, and the rest and children are not allowed to use the correct, reality-based pronoun in school, for a trans teacher or a trans child. (As if there are trans primary children! What the hell were people thinking?) How many have of these policies have I read? Dozens.
Young children do not misgender anyone. Misgendering itself is made-up, politically-motivated term invented by TRAs. If the children called Mary 'sir' as their norm, that is not misgendering. Calling Mary 'miss' would be incorrectly identifying Mary's sex. It would be taking part in an adult-based lie that they do not understand. The suggestion that children referring to Mary in line with observed reality is somehow wrong - seriously, think it through. That's insane. We are supposed to teach children the truth, and it gets no more real than the reality of their sexed bodies.
Children are not mistaking the teacher for Mum if they call her that. They are simply used to saying Mum when they need help/attention/etc. At no level do they think the teacher is Mum. Your example makes no sense.
The point of calling Mary 'miss' is to confuse children about the nature of Mary's biological reality.
Where your last paragraph comes from I have no idea. I never said that a trans-identified male needed to dress up to access children. My male colleagues undertake the same background checks and safeguarding training that I do.
A trans-identified male who is comfortable with children calling him 'he' and 'sir', in line with reality, poses no extra threat, either. Indeed, it is a refreshing break with societally-enforced gender norms to see a man in a dress. As long as the children know he's a man. And their language is not policed in line with TRA-influenced policy.
Trans people deserve protection in employment, including in schools. This does not give them - or you - the right to demand that children participate in the fantasy that they have changed sex.