A Canadian study found that eating more fruits and vegetables was associated with better academic performance among high schoolers.
A second study looked at brain performance in children 6 to 14 years old before and after one year of taking a multivitamin/mineral supplement. The results are hardly surprising: there was a significant improvement on two different attention tasks than kids not taking the supplements.
Ironically, what are the most commonly prescribed medications for school aged children? Of course they are medications for attention deficit. How much more sensible would it be to supplement these kids with multivitamins than to "supplement" them with pharmaceuticals?