Lurky and Yadidaye... if you look at the consultation document that XXSex linked above, the proposal appears not to relate to teachers actively supervising the toilet area. Rather, that the open plan design allows for "passive supervision".
I dont know what the definition of "passive supervision" is, but I suspect it doesn't mean that a teacher is assigned to stand at the toilets- more that the prospect of a teacher walking past may be deterrent enough to stop bullying.
Now, having been a teenage girl in a mixed school, with teenage boys (the majority of whom grew out of the offensive "lets be crude while we wade through the hormonal surges stage of life" and became decent men) I wouldn't rely on passive or active supervision in the loos.
Upthread, teachers have commented on having their toilet door thumped by boys, who think that "periods" are somehow funny or weird.
Boys wont be deterred from that behaviour in the short term.
But, while they're learning, it's not fair for girls to have to deal with their own body changes and the immaturity of males at the same time.
So, let girls have some dignity and privacy in single sex places.
Trans girls and boys can go to a third gender neutral place (not the disabled loos).