Well, here we are, with a whole new thread!
www.mumsnet.com/talk/womens_rights/4713725-please-help-gender-not-sex-on-a-school-protected-characteristics-poster-just-spoken-to-the-head?page=10&reply=123546552
Thank you all, so so much for the support you have given me.
I am currently working on the draft of the message I want to send.
Long story short, DD's primary school displays posters by No Outsiders, showing the 9 protected characteristics of the Equality Act. Sex as been replaced with gender.
I have raised this with the Head, the assistant Head, and the Deputy Head (DepHead, as DH just doesn't work here). I have been reassured that sex is synonymous with gender in law, and, through an email stream, No Outsiders have reassured the school that it is deliberate, gender is easier to teach children than sex, especially as some will snigger at sex.
Oh, and teaching that gender is the difference between boys and girls, it makes it easier to then explain gender reassignment later on.
I'll put the message from the DepHead and No Outsiders here, and also my reply, in its current state.
Thank you all again!