As Maud said, most high schools are single sex. In fact, in our area, most junior schools are also single sex.
Dd1 will be starting school in Sept. That school is mixed for first 3 years and then the boys move out to the boys school and girls stay put.
The high school their names are down for (yes, dd2 is just 14mths old!) is the school I went to. Single sex, public, Catholic... The other nearby option is single sex, private, Catholic...
This is pretty much a non answer -- but I think it depends heavily on the child, the family, and the school.
I agree with this. I went to an all girls high school. It was a good choice for me. I never got the impression that the boys at the all boys school benefited in the same way.