I went to two girls' secondary schools: one a rather posh (but selective, and academically strong to this day) boarding school in England, and the other a public school in my home country, a school that was extremely selective and also very diverse, ie pupils of all races. The two leading schools in my country were single sex, one for boys and one for girls. Both had a very high standard, as high or higher than in the UK. Both were later made co-educational and still are today.
I loved being at an all-girls school. I have always been a quiet, introverted person and I hate making my voice heard, always have. I would have been trampled under in a mixed sex school.
Both of my girls' schools had a strong STEM approach but really, STEM is not the be all and end all. Children who are more artistic, more "dreamy", as I was, need strong encouragement , and I'm pretty sure I'd have been completely tongue-tied and lacking in confidence had I had to cope with boys, a shadow of myself.
To me, it's a matter of providing a structure in which a more sensitive mind can grow, just as we might put a fence around a young tree; it helps that tree grow strong, which is what happened to me. As an adult I became fully able to cope with men and have had a very rich professional life.
Both my children went to mixed schools. My daughter didn't want to go to my old UK school, which would have been my first choice. She joined my son at his school. I am totally in favour of girls' schools as an option.