Here is a New York Times article about a recent study performed by the British government.
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Some Food Additives Raise Hyperactivity, Study Finds
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This was a high quality study and is good evidence that a diet high in preservatives and artifial coloring leads to more hyperactivity and inattention in children.
Should we all run to our pantries and throw away our junk food? well maybe....
the fact is that moderation is always the key.  We would be healthier by eating more vegitables and fruit, but public health evidence suggest that taking ALL preservatives out our food supply would lead to more illness related to consumption of spoiled foods.

The take home message is to reduce and limit.  Eat more Fruits and Vegitables.
You may find your childs behavoir and grades improving.