From American Psychological Association Journal - "Results from the highest quality studies ... do not support the view that SS (single sex) schooling provides benefits compared with CE (co-ed) schooling."

In this article, authors Erin Pahlke of Whitman College and Janet Shibley Hyde and Carlie M. Allison of the University of Wisconsin—Madison share the results of their analysis of a wide variety of studies of the impact of single-sex v. co-ed learning environments.  Pahlke, et al. concluded that, despite persistent claims to the contrary by advocates of single-sex education, there is no evidence that gender-homogeneous classrooms lead to improved learning outcomes for students.

Given the self-evident limitations inherent in an environment that by its very nature excludes one gender or the other, this lack of demonstrable learning benefits is, it seems to me, particularly problematic for those promoting this educational approach.