What's worrying to me is the number of people saying "ask the mum/ask the school what words they use to explain it, then use the same explanation with your dc."
Where is the critical thinking about this??
So some other family or the school has signed up to the backward notion that gender roles are innate and edifying, being a girl or a boy is determined by what you wear, and there's such thing as "feeling/thinking/acting like a girl", as opposed to being an individual with a unique personality. And therefore the 'right' thing for you do to is to unquestioningly sell your impressionable kid the same line of sexist bullshit, shaping his or her own attitudes toward what it means to be a girl or a boy, and how boys and girls 'should' think and behave, all in order to avoid causing offense by deviating from the other parents' / school's narrative about gender?
Fuck that.
I would be approaching the school and saying clearly that if there is indeed a child identified as transgender in the class, I fully understand the teacher and school's obligation to be supportive, inclusive and nurturing to him/her, but I expect them to do so without validating or reinforcing sexist stereotypes at school, without suggesting to the children that it's possible to change biological sex, without using 'gender identity' in place of 'personality' to describe interests and characteristics, and without making any child, particularly girls, feel that they should be more concerned with someone else's feelings than they are with their own boundaries, when it comes to things like who uses the bathroom with them.
And I'd be telling my child that different people believe different things. We believe that being a boy or a girl has nothing to do with what you can wear or what you can be interested in or what you can do, unless it's something you do with the boy or girl / man or woman parts of your body (like having a baby). Other people believe that girls should have one kind of personality and boys should have another. Lots of people used to think that way, and thats why women used to not be able to vote and men didn't know how to change nappies. I know, can you believe it?! Thankfully, a smaller number of people think that way about what men and women should be allowed to do, now. Most people know that boys and girls can like whatever they want, and men and women can grow up to do any job. But if a family believes boys should be one way and girls another, and they have a boy who wants to wear pink and play with dolls, they might believe that means the boy is really a girl. That's not what we think makes a boy or a girl, but it's okay for people to believe different things and the important thing is that we're kind to others whether their beliefs are like ours or not. And I would be reiterating to my dc that no interest or favorite colour or outfit or personality trait will ever change the fact that they are a boy / girl, or make them more or less of a boy / girl. And equally the fact that they are and always will be a boy / girl should never limit their interests, and anyone who makes them feel like it should, is plain wrong.