I can't honestly tell a whole lot of difference between the people I know who went to single-sex or co-ed schools. There's no telling what will be right for a particular person at a particular time in his or her life based on generalities. I know people for whom a girls' school was the ideal place; I probably would have been much happier in one in middle school.
However, if your object in sending your daughter to a girls' school is to protect her from the opposite sex, it ain't gonna happen. Teenage girls are interested in guys; if they aren't at school, they'll find them elsewhere.