NYT and WP articles from September 2011, and Strauss on a meta-analysis. These articles suggest that advocating for single-gender classrooms will be more about social issues than academic performance.
Where it’s happening now.
The ACCES web site – American Council for CoEducational Schooling
The NEA page on single-gender education
The greatschools.org page on single-sex schools (which is technically different from single-gender classrooms.)