I understand what simi63 is saying and I think she has made a good point. I also think that parents, with consultation of their child's teachers, might make a decision to send their child to a sex-segregated school if needed. Some girls become inhibited around boys during adolescence and only excel when put into an all girl class. Perhaps this is the same for boys too. I don't know.
Also, quite frankly, some teachers favor the boys or men in a coeducational situation by calling on them more, having higher expectations, and this is communicated in their behavior toward the students. Things are changing in the U.S. but when I was a girl, sometimes we were meant to feel less than the boys in academics, sports, everything.