One way to see how one reacts is to cleanse out all the top allergens and cleanse in general.
Wheat, dairy, soy, eggs, some say corn, red meat, yeast, processed sugar...gluten possibly...with attention disorders and delays I have heard it can really help some.
Then you add non likely offending foods, and then expand slowly over time to see if there is a change. The cleansing time seems to vary with opinion...my experience was that 3 weeks was a good amount of time.
I ate brown rice, salad greens, for a week, and added a new fruit or vegetable every 3 days after. After the 3 weeks, I tried the offenders. I found how things are cooked can make a difference as well. I.e. I do well rotating hard boiled, or eggs baked in things....but when I have invertase, or plain egg white (found in some GF cookies) or mints, I will get a really bad headache. So I try to be careful about "treats".
Check out elimination diet and paleolithic diet.
Anyone can have allergies ...or sensitivities. Some foods some can rotate, others not a good idea at all.
It helps if one keeps a food diary. I found I can rotate nightshade, but I just break out and itch with baker's yeast. Second time around on wheat was really hitting me so I don't eat wheat, and think I could be fine with gluten (had numerous celiac blood tests all negative and no reaction so to speak of from rye or barley), but with my older son's sensory processing issues and speech difficulties, I am going GFCF with him and the house. I admit I will drink coffee with some real cream now and then, and occassionally eat white cheese, such as mozarella or farmers.
Sunflower seeds and many nuts, if one is not allergic can provide minerals.
Some take vitamin drinks or mineral mixtures such as Cal Mag blends. A nutrition almanac can be a useful tool to compare what is in foods and their amounts.
So...cleansing, elimination diet, food diary, and nutritional almanac,...would be what I would recommend as reading material to help one find out offending foods and create an understanding of how to supplement.
Hope that helps...good luck with your choice to eat healthy. Wish the schools would be more aware.