Schools as far as I can tell (I'm ks1-2 so it doesn't come up much) aren't explicitly told about affirmation models. What it does say is this:
^^We are aware that topics involving gender and biological sex can be complex and sensitive matters to navigate. You should not reinforce harmful stereotypes, for instance by suggesting that children might be a different gender based on their personality and interests or the clothes they prefer to wear. Resources used in teaching about this topic must always be age-appropriate and evidence based. Materials which suggest that non-conformity to gender stereotypes should be seen as synonymous with having a different gender identity should not be used and you should not work with external agencies or organisations that produce such material. While teachers should not suggest to a child that their non-compliance with gender stereotypes means that either their personality or their body is wrong and in need of changing, teachers should always seek to treat individual students with sympathy and support.
You should work together with parents on any decisions regarding your school’s treatment of their child, in line with the school’s safeguarding policy and the statutory guidance on working together to safeguard children.^^
Two important points there:
That stereotypes should not be used to affirm gender identity.
So if Billy says, I'm a girl because I like long hair and nail varnish, a good practitioner will point out that gender non conforming people aren't necessarily the wrong sex.
However, if Billy says I'm a girl because I feel like one sensitivity is expected to be used and a new name etc. may be agreed with in consultation with the parents.
The teacher then has the awkward tightrope of having to respect the child's wishes without affirming their political view. Teachers, thankfully, do have some experience of this, such as with religion, but I imagine some less experienced teachers or those with an agenda going off piste.
The second point is in guard with safeguarding the school needs to work with the parents, can't promise to keep secrets etc. Do in practice how 'affirming' the role the school takes takes the lead from the parents.