There's plenty of girls-only and boys-only schools around here, in fact I nearly went to one myself. In the state system, single-sex schools generally out-perform mixed schools by far (in the private sector, like my school, there is generally no difference).

My school actually used to be a boys school, it's only been co-ed since 2001, so there are no more than 20 girls in each year. I think there's 14 girls in my year to about 100 boys.

Personally though, I think a strict school uniform has a better effect on keeping boys and girls apart during school than separation.