it's not the fat content that made it unhealthy, it's that it was COOKIES. Yes, cookie crisp cereal says it's whole grain, but it's very very little whole grain and sugar is one of the first few ingredients. Basically they are little cookies.
Why not email the teacher and ask for a list of ok snacks since this seems to be an issue?
Some ideas - bagel, pretzels, cut up strawberries in a bag, blueberries or grapes or an apple or pear he can chomp (doesn't need to be cut), crackers, graham crackers, carrots or cucumbers or celery or green pepper with dip in a little container, kix, chex, or cherrios are good cereals, fig newtons might be allowed but I would check, granola bars might be allowed, but again depends on the rules.
I also wanted to add that at least in our school district (Plainfield 202) they can't have anything that is not whole grain or fruit/veggies, low in sugar, under 100 calories, and healthy. Our school was the pilot school for the wellness policy which all schools in the USA had to have in place in order to still receive federal funds. So, it may not be his teacher, but the district that set the rules.
N.