He was asked to draw his feelings, his feelings were to take a gun and kill teachers. Behavior issues that can be solved by parents are generally caused by those parents in the first place; the crummy hands-off "let the system raise my kids" parenting we see all too often.
Their first reaction was correct; get the story and make sure everyone's kids and the staff are safe with this kid in the school. The follow-up of arresting him was NOT.
Sorry, if I have to choose between being cautious or assuming other people are being good parents... I'll choose caution because I see bad parents every day out in public.