When you put girls and boys in single sex classrooms, girls get equal treatment with their peers, have more leadership roles, and enjoy a rise in self-esteem. They also say that they prefer single sex classrooms. Boys are more willing to learn through cooperation and teamwork. Because their learning styles and learning tempos are different, we see both boys and girls benefiting from classrooms slanted toward gender differences.
Research on the advantages of single-sex classes and schools has typically been done at the middle school and high school levels. For younger children, much of the research is simply anecdotal tales from individual teachers. Some schools have found rising test scores in single-sex classes and schools while others have found that these programs run smoother with fewer discipline problems. Many reports, however, show that single-sex classes are usually no better than mixed-sex classes.
At the present time, single-sex education is a fairly hot topic. Much of this interest stems from a 1992 report "How Schools Shortchange Girls" by the American Association of University Women (AAUW). Since then, several states have set up single-sex classrooms and schools. You can read more about this issue by looking at the previously-mentioned AAUW report and these sources: Failing at Fairness by Myra Sadker and David Sadker, the AAUW report "Separated by Sex," and "Boys and Girls Together" an article in the American School Board Journal, December 1998.