This would be better as a discussion question because many people will think that different things cause this disparity.
I would argue that the main cause of the disparity is that boys are socialized to be more aggressive and less obedient. They tend to think that it is less "cool" to just be good and listen to the teacher and do their work. This leads boys to do things like acting out in class while girls tend to be more likely to be good.
I think that boys are also slower in developing their verbal abilities. Because of this, they often come to feel that they are not as smart as they really are. This leads them to be less interested in school and therefore to do badly even after their verbal abilities improve.