Thankfully I anticipated this and sent my son to school with his own snack, or else he would have had nothing while everyone else ate their group-effort snack-- including the ingredient that he brought which was dumped in with everything else. Ugh. Allergens are hidden in lots of things, and it can be tricky, but here's a hint: PEANUT Butter Cheerios contain nuts.
p.s. As a teacher, wouldn't you leave out the offending ingredient so everyone could participate?