There are a number of reasons why single-sex classrooms might benefit students.
First of all, such classrooms are said to be beneficial to girls. It is said that teachers do not treat girls equally in mixed classrooms. It is also said that girls tend to be less willing to speak out in class when there are boys around. Researchers argue that girls are more likely to act in less assertive ways when boys are around because they feel that that is what is expected of their sex.
Second, there is the idea that a single-sex classroom would be free of the sorts of hormone-driven behaviors that happen in mixed classrooms. In other words, students of both sexes could concentrate on learning rather than on trying to impress the opposite sex.