Why does the concept of single-sex education continue to be considered? There are many people on both sides of the argument about whether single-sex education is beneficial for children and teens, and you can find research to support both perspectives.
On one hand, advocates say that students in same-sex schools are less distracted by the opposite sex, girls demonstrate more interest in math than their co-educated peers do, and students in same-sex schools score significantly better on cognitive tests. This would suggest that individuals focus on schoolwork more or have a broader range of interests available to them and that the academic options are less gendered-limited.
The other side of the argument includes concerns that if teaching patterns are altered to "fit" one gender, stereotypes will be reinforced and the differences between the genders will be enhanced, ignoring individual difference. This perspective is related to the fear that in a same sex educational setting each gender will reinforce the cultural expectations they perceive - girls becoming solely concerned with their looks and boys becoming solely concerned with their strength.
We do know that any education must be well-implemented, take into account individual differences, and "engage" students at many levels. Whether that education is public or private, same sex or coed, seem to be less important than how motivated students are to participate in the education provided. There are poor schools everywhere.
Having two daughters in a single sex school I have to admit I have a pretty strong bias, but I also see a tremendous amount of difference between the coeducational elementary school education they received and the single-sex middle and high school educations they are currently receiving. Walking around campus after school the difference is particularly striking. There are groups of girls preparing for, or competing in, sports, there are practices in progress for plays, orchestra, and academic competitions, as well as clubs meeting, community service activities, and grabbing an occasional nap. The clubs include subjects that are definitely not gender-determined. The include subjects as diverse as robotics, manga/anime, knitting, trivia, and garage bands. The girls excel at many different things, none limited by their sex.
This same sex environment encourages these girls to try everything and not be afraid to fail at anything. Just that ability to try something new promotes health and wellness in many ways. Physically, kids who do not fit the social "norm" of attractiveness are less focused on their appearance then they might be in a coeducational setting, and much more likely to participate in sports. Academic kids are able to excel as well, finding they are less concerned by being called "nerds." Kids in same sex schools also tend to be more comfortable with their bodies, as well, allowing more open discussion about the changes associated with puberty and growing into adults.
Nothing fixes everything, but I suspect that same sex schools support student health a little bit more than coed schools, particularly if classes are smaller and there are counseling and support services available.