An extensive review of relevant research has demonstrated that the more physically active schoolchildren are, the better they do academically. 
Researchers analyzed 14 studies, ranging in size from as few as 50 participants to as many as 12,000. 

All of the studies involved children between the ages of 6 and 18.
According to the authors:
“Physical activity and sports are generally promoted for their positive effect on children’s physical health; regular participation in physical activity in childhood is associated with a decreased cardiovascular risk in youth and adulthood.
There is also a growing body of literature suggesting that physical activity has beneficial effects on several mental health outcomes, including health-related quality of life and better mood states.
In addition… there is a strong belief that regular participation in physical activity is linked to enhancement of brain function and cognition, thereby positively influencing academic performance.
There are several hypothesized mechanisms for why exercise is beneficial for cognition, including:
(1)  Increased blood and oxygen flow to the brain
(2)  Increased levels of norepinephrine and endorphins resulting in a reduction of stress and an improvement of mood
(3)  Increased growth factors that help to create new nerve cells and support synaptic plasticity
… The increasing pressures to improve academic scores often lead to additional instructional time for subjects such as mathematics and language at the cost of time for being physically active. Given the suggested relationship and the ongoing discussions on the replacement of physical education lessons by academic subjects, we aimed to review the evidence on the longitudinal relationship between these two variables…
To summarize, the literature provides inconclusive evidence on the positive longitudinal relationship between physical activity and academic performance. However, there is a strong general belief that this relationship is present, and research in this area is ongoing.”

Exercise and Academic Performance
Keeping kids active at school is a superb way to increase learning, focus and even test results. As many of you reading this have likely experienced, if your mind is feeling cluttered or you’re having a mid-afternoon slump, a brisk walk or a quick workout can give you a renewed sense of clarity and focus.  This is certainly true for kids too.

Two years ago, ABC News reported on a special program being implemented at Naperville Central High School, where students could take part in a dynamic gym class at the beginning of the day, and had access to exercise bikes and balls throughout the day in their classrooms. The results were astounding. Those who participated nearly doubled their reading scores, and math scores increased 20-fold!

Research has shown that after 30 minutes on the treadmill, students solve problems up to 10 percent more effectively.
Although it’s becoming more widely known that physical activity has a direct result on brain function, many schools in the US are removing rather than improving their phys ed programs… This means it’s up to you to encourage your child to stay active after school and on weekends in order to reap the wonderful brain-boosting benefits that exercise has to offer.
 
Recource Mercola.com