The link below is a video on reschearch that was done on 5th-7th graders on how well they did academically compared to their fitness activities.  The children that were more physically active got better grades in school than the sedentary children.  Also, as the children improved physically there grades improved as well.  This is because exercise boosts the cells in the brain improving long term memory.  Science is proving more and more that in children and adults who get regular exercise the benefit is not only physically but mentally as well.

http://www.cnn.com/video/#/video/health/2010/03/04/hm.fitness.and.academics.cnn