I went to an all-boys school: I liked it and did very well there, but I suspect single-sex schools were more common then than they are now. I think the temptation to show off to the opposite sex all the time can prove too much of a distraction in mixed schools, and if you do encounter relationship problems with your girlfriend, it's probably better if it can't spill over into the classroom.
What's wrong with realistic experiences like learning to interact with the opposite gender when young and going to school? Or is it better to shove them into the grown up world with no previous experience?
Hey Wait! Teenagers Learn???
Separate but equal is inherently not equal. Segregation is a thing of the past and I think we should know better than that by now. I don't see how separating a child from an entire gender could contribute to their education in any way. Also, same-gender schools are binary and don't take into account the fact that not everyone fits into the gender binary. It excludes people who aren't cisgender.
There's an age old belief in the world of private education that girls learn better in single sex schools and boys do better in co-ed schools. Go figure.