It was bullying, no contest. The teacher in question had apparently repeatedly misgendered the child, in a manner calculated to cause distress & possible derision from other pupils.
However, this could have been dealt with by following the school's disciplinary policy, on the grounds of harassment. Mermaids could, no doubt, have assisted them with escalating the complaint when it was not asnwered correctly when they first submitted it to the school - I am sure they are well versed in the intricacies of the law in this respect. The police did not need to be involved.
I think this is indicative of the attitude of the vocal extremist minority of GRAs who are, sadly, leading the debate at present. I am glad it's been picked up by some of the national mainstream press.