A recent study linked behavior problems to dietary quality during two key developmental time frames. Unhealthy early dietary patterns, a mother's prenatal diet and a child's diet from birth to age 3, were independently related to the risk for behavioral and emotional problems in children. These diets are both, of course, well before a child attends school.
I’ve had some parents over the years ask us to not serve food with additives, preservatives, and artificial coloring. Frankly, I do not want to eat many center-of-plate, low cost foods (which is what public schools have to serve) that do not have preservatives. Shelf life is important and food deliveries only come once a week. Food borne illnesses are serious and we serve a susceptible population. Additives are not inherently wrong. They emulsify, leaven, enrich and improve our food. Artificial coloring may be a little shadier, but know that many old school standbys have been improved - our strawberry milk is now colored with beet juice.
I know, it's complicated. Child behavior is complicated. Childhood obesity is complicated. Just don't be too quick to blame school food.