If I remember correctly about this incident, it wasn't only that the school was warned about the child's behavior from previous incidents and from earlier that day and it had been ignored, as you have already received a comment about, but I believe the mother also called the school to tell them her gun was missing and she believed the child had it and they didn't do a thorough enough search for it - I believe they only looked in his backpack and not in his desk or jacket, etc.