1. Students of the opposite sex can be a distraction.
Many teenagers in high school will be distracted by members of the opposite sex who they find attractive. Students (mainly teenagers) will prioritize getting a girlfriend or boyfriend instead of focusing on academics, which is more important for their life than a high-school partner.
2. Single-gender education can broaden the perspectives of learning for students.
For example, girls who want to study science or math often face the pressure of having to compete with mostly males. Girls who go to all girls schools no longer feel this pressure, since they are only competing with other girls. Same thing goes for guys who want to pursue more feminine subjects, such as poetry or other arts programs. In a all boys school, guys can more easily achieve their dreams of becoming successful in those areas because of less competition.
3. Boys and girls learn differently, therefore, creating two different environments of education for them will enhance their learning experience.
I think this one is pretty self-explanatory. Single-gender schools have teaching methods which can be applied more successfully to that specific gender, and therefore, improves the quality of education for both genders.
Single gender schools are very good for children and for teachers due to the differences in the speed and time of development of children at that age. While the ability of the two genders are not particularly different in maturity, as children, they differ substantially until a few years after puberty.
Single gender schools can foster the best learning. This is especially true for young women when they hit the teen years and are inclined to act less smart so that they can attract the young men. With opportunities to socialize built in, a single sex school could be very beneficial.
Single gender schools are not complementary to the common good. When children or even young adults are sent to these schools, they grow up with either an extreme aversion or extreme attraction to the opposite gender. When they are released into society, this causes problems as the children do not know how to act amongst people of the opposite gender.
Single gender schools are a thing of the past in our society. They were once thought of as effective, but hold little use or value in toady's day and age. Students face an extreme amount of diversity in life and need to interact with both genders while in the education system.
Single gender schools are not complementary to the common good. I think that single gender schools would be a bad idea. People and students have to learn to communicate with each other and school is a good way to learn from everyone. Girls and boys have different approaches and ideas to subjects in school and to separate that would be a mistake.