I may have missed a thread, but there's some new draft guidance from the EHRC on schools out for consultation: www.equalityhumanrights.com/guidance/technical-guidance-schools-consultation
Notably, it gives this admirably clear example:
A secondary school is considering how to provide appropriate changing facilities for a trans pupil who identifies as a girl. The school must provide single-sex changing facilities and showers for children who do physical education at the school. It is therefore not permissible to allow the pupil to use the girls’ changing room. The school decides that the pupil must use the boys’ changing room. This is unlikely to be justified and so is likely to be unlawful indirect gender reassignment discrimination. A suitable alternative might be to allow the pupil access to a single-user, lockable changing room, which might be a staff changing room or a mixed-sex accessible bathroom.