I know of a number of schools , my daughter’s former school included, who have obtained legal advice on the issue. The sticking point has been the EHRC Technical Guidance to Schools which is currently being reviewed by the EHRC but in its current form makes it clear that to not treat a child as the opposite sex is discrimination under the PC of GR under the EA.
This guidance was published in 2014 and has not been updated and was very heavily lobby group driven. It is only in the past 18 months or so that it has been questioned. Legal opinion on the discrimination question has, in my experience, been based on this guidance.
Conveniently, these legal opinions obtained by schools tend to ignore parental responsibility, children’s rights to appropriate healthcare etc and only focus on discrimination (which may well not hold following whatever update the EHRC makes).
Schools should also have a look at the NHSE MindEd modules which were issued yesterday and take a cautious approach towards social transition. While not DfE guidance, in the absence of such guidance, the module aimed at the education sector might open schools’ eyes a bit that it’s not as straightforward as affirm the child as soon as the request.
https://www.minded.org.uk/Catalogue/Index?HierarchyId=0_59819&programmeId=59819