I went to an all-girls high school (3000+ girls). At the time I wasn't keen on it, but since then I realized that many of the girls were liberated from attracting (and being attracted by) a boy. We focused on academics more than we would have done normally.
Now, it's about 50/50 split where I live. The public high schools are co-ed and the Catholic high schools are segregated. There are about the same number of students in both kinds of schools.