"But if they aren't girls, why do they want to go to a girls' school?" is the first silly question being knocked about. If you were starting out afresh as a prospective first year pupil and were male you'd be rather unlikely to apply, but if you are five years into your time at a school and about to sit your TGAUs, swapping schools and having to leave your friends and familiar spaces behind as well as dealing with your own gender vertigo may all be a bit much to cope with.
Persistent misgendering damages lives in many ways including those all-important-to-schools exam results, so it's in the school's enlightened self-interest to not have gender variant pupils under more stress than needs be the case.
It also has the fringe benefit of inculcating in the pupils a sense that they aren't "girls", the subtly belittling diminutive which gets used for women so much further into adult life than is the case for boys. So a bit of a boost for the cis kids too.
The other bloody stupid question being punted is: "but what shall they call the school?" This one carries on into extended drivel which boils down to:
"If it's now a school for girls and people who we thought were girls right up to the middle of their A levels and then found out we were wrong, and all of a sudden we've decided that people who are legally able to leave home and live independently, pay taxes, get married, have children or join the army and die in a war overseas are now also allowed to have an opinion as to what name they should be called, well, the sign on the front the school will have to be changed because PC Police."
I can't help but think that the schools I attended between 10 and 17 both had signs on the gates with "Saint" in them, and there was precious little in the way of saintly behaviour amongst the attendees be they staff or student. I suspect the students of AGSG will cope...