Personally, I feel that single-sex schools could actually have negative effects in the long run. Eventually, girls will need to be integrated into professional society with men, so we may as well get used to competition and occasional bias (in both sexes' favor). As a girl, I have never felt inferior or like my potential has been limited due to my male classmates. Some teachers may call on guys more in general (maybe they don't want to pressure girls, which is understandable, although wrong). However, I raise my hand and answer plenty of questions. But I never have been shy that way, and I don't care if my answers or questions sound stupid--we're there to learn, right?
Some claim that girls are too concerned with their images and appearance when around boys, and that they are easily distracted. Well, I have little sympathy. What you focus on is determined by your priorities and values, and getting rid of the stimuli won't cure the disease, just put it off. And social life is fine in a school setting, it's part of education, too (interacting with others) -- everyone has to learn to deal with juggling their life. Although girls might spend less time getting ready in the morning for a single-sex school, I don't think that's a reason to separate the sexes. And I thought we had learned from the Civil Rights Movement that separate is not equal!?!
Unfortunuately, I believe that encouraging single-sex education is like admitting defeat for women's equality. That may sound harsh, and I don't think anyone intends that to be, but the idea does promote a feeling of inequality among girls, at least subconciously, I think. Girls have to realize they can and must learn to compete and work together with boys at an early age. And teachers have to make a concious effort to not treat girls more gently in class (by not pressuring them to think, etc.). We, as the next generation, have to instill proper values in our children, so that boys and girls can have equal confidence in their equal abilities. I don't mean to condemn single-sex schools, I just think that coed is all around better.