I recommend you to read an interesting article written by Dr. Russell Blaylock, an American neurosurgeon, author and lecturer.
Dr Blaylock describes the case of a child with disruptive behaviour in school whose mother was told the child should be prescribed Ritalin if he was to continue at school.
The child was tested for food intolerances with positive results. Once the foods were removed from his diet his behaviour changed completely and he was able to return to school. Please read this : ADHD or Food Intolerance