Both things are related with each other. When you become smart you can run fast.
To keep your weight normal adopt these things.
Watch your soft drink intake. Always prefer simple water. You can take diet colas only once on the weekend.
Cut back on fast food & eating out. Both have lot of calories. Never take french fries. Do not eat except fruit in between two meals.
Get moving, watch less T.V.
Walk, run play even after school hours.
Inshallah you will be okay.