It's probably something they eat all the time at home, and in the morning rush they just got absentminded and threw it in the lunch bag. Or maybe a kid slipped it in. Or maybe grandparents were visiting and packed lunch and aren't used to the rules and forgot. I've almost sent granola bars with nuts before but always caught it at the last minute. I understand being upset, and I'm sure the parents feel terrible, but I seriously doubt they packed peanuts out of spite or willful disobedience. I hope no one's child got hurt, and if they didn't, I would really let it go this time as long as the teacher talks to the parent. If someone's child had a reaction I'm sure they feel truly awful.