There's an interesting article from yesterday's New York Times about a school in the Bronx that's experimenting with splitting up the girls and boys in its fifth grade class. The goal is basically to see what happens in relation to behavioral improvements, testing, etc. It turns out that there's even a national movement going on to split up boys and girls in public schools. There are more than 400 such classes around the country, thanks to a federal regulation passed in 2004 that gave schools the right to do so.
There isn't enough test data yet to suggest that the move has academic benefits, but teachers and students alike have good things to say so far. Both male and female teachers feel that it's bonding them to their students more and that the students are bonding to each other.