The school has a statutory duty to follow the Equality Act 2010 which 'requires all schools in England to eliminate discrimination on the grounds of sexual orientation and gender reassignment, advance equality of opportunity and foster good relations between different groups of students'.
This is what happens when single issue lobbying orgs advise on equality law.
The Public Sector Equality Duty, which forms part of the EA, actually says the school must:
(a)eliminate discrimination, harassment, victimisation and any other conduct that is prohibited by or under this Act;
- and this applies to ALL the protected characteristics (except marriage/civil partnership which only applies in employment), not just sexual orientation and gender reassignment.
(b)advance equality of opportunity between persons who share a relevant protected characteristic and persons who do not share it;
(c)foster good relations between persons who share a relevant protected characteristic and persons who do not share it.
Further, (3)Having due regard to the need to advance equality of opportunity between persons who share a relevant protected characteristic and persons who do not share it involves having due regard, in particular, to the need to (among other things):
(b)take steps to meet the needs of persons who share a relevant protected characteristic that are different from the needs of persons who do not share it;
Such as maybe providing single sex toilets for girls who are very likely to be starting their periods at around this age, and not making them share sinks with boys.
It's useful for them to know what harassment is, as they have a duty to eliminate it. It means engaging in unwanted conduct related to a relevant protected characteristic which has the purpose or effect of violating someone's dignity or creating an intimidating, hostile, degrading, humiliating or offensive environment for them.
I'd say removing the girls' toilets and making them share with boys fits that description.