my school has never had a policy where we exclude parents, we have always included parents at the earliest stage.
That's great but it's not the case everywhere. Our school's policy had a line - lifted straight out of KCSIE, which was in turn inserted at the behest of Stonewall (the CEO boasted about it on twitter) - that stated that a child stating that they were trans wasn't in itself a safeguarding issue, you can affirm, no need to report to anyone, no need to involve parents. Some schools interpreted this as a duty to socially transition at school. Ours interpreted it as a duty to only inform parents if it would "increase the happiness or well being of the child". This seems to turn safeguarding and working together on its head, which is that you always do tell parents unless such a disclosure would put the child at risk.
Yes Cass says that for some children social transition will be the right decision, but she also says that should be a clinical decision, should be vanishingly rare, and basically should never happen at primary level at all. This means that the policy not to inform parents cannot stand, as how can you find out if there has been a clinical decision unless you contact them.
I think a lot of schools forget that they are not counsellors, doctors or parents because they are - unfairly, IMO - expected to be all these things so regularly these days, without the appropriate expertise. Add in teachers who feel they should be activists or "providing a safe space/rainbow family" for students with parents who are perceived to be unsupportive, abusive, right wing or religious and the issues really start stacking up.