There is now a recognized body of research which posits that boys do learn differently from girls. Read books such as Boys and Girls Learn Differently!
by Michael Gurian to understand that line of thinking. Teachers in a boys' school understand how a boy learns and as a result are quite successful in implementing the special teaching techniques required to achieve optimal results.
2. The focus is entirely on boys.
When you don't have to plan curriculum, lessons and events to include both sexes, you can focus on boys' needs exclusively. Young men enjoy different kinds of activities than girls do. They grow at a different pace than girls do. A boys' school allows a boy to remain a boy for as long as he wants and needs to.
3. The social pressures are much less stressful.
Boys mature later than girls do. Learning how to cope with and relate to girls on a daily basis in a coeducational setting causes added stress in those early adolescent years. That's exactly the time a boys' school is able to jump in and build confidence without the social distractions inherent in a mixed setting.
4. Boys become comfortable with non-traditional subjects and activities.
In a coeducational high school, boys often shy away from joining activities such as choir and orchestra. To do so would make be considered unmasculine. In a boys' school, ou can't have a choir or an orchestra unless you and your classmates pitch in and make it happen. Breaking down the stereotypes society has imposed on what are considered appropriate activities for boys is one of the many things boys' schools do very well.
5. Boys learn that there are many routes to manhood.
Again, traditional stereotypes can be dismantled and replaced by a more thoughtful approach to maturation. Not every boy can be a star athlete or a math whiz. Competition is fine and to be encouraged. But boys can also learn that the strong bonds of friendship, teamwork and social interaction are what matter most in later life.