I went to an all girls private secondary school.
I foudn it a veyr positive experience. I am often shocked by some of the thigns people say they/dds have to endure in secondary.
For me it was basically an environment where sex was irrelevant, in that there was no sexual tension, no 'boy' or 'girl' subjects, no mansplaining, no patronising, no competing for time with boys, no sexism.
There was also lots of strong women demonstrating professional lives, both in the teaching staff and in the women who came bakc to share about their careers.
I have 2 dc at secondary and they are at grammar schools. The grammar schools accessible to us were both single sex, one for boys and one for girls.
After much though I have sent ds to the boys and dd to the girls and hope it is a positive experience for both of them. The advantage is they share school transport, and a sixth form.
Seeing ds and his mates I like the fact that girls and all that involves isn't really yet on their agenda, aged 14. Also the school is very positive with the boys and there is less of the 'disruptive boys' feeling going on. No 'girls' subjects, they all get stuck into cooking and art etc.
dd is in her first year at hers, and while there is lots of positive stuff, I realise she walks to the beat of a slightly different drum to the other girls. There are only 4 girls in her year who chose to wear the uniforms trouser option, and only 2 in her year with short hair. She has found 2 close friends, but it took a while, and her friends from the train are both boys from the boys school rather than girls from her school. But she is a quiet, gets lost in the crowd type, who might and disappeared in a noisy classroom.
I still think she will benefit from the positive for girls environment, but wonder if we should have gone for co-ed. (but then not grammar, so difficult choice)
So it can work for both boys and girls. Probably. I hope.