I mean, if you're a member of law enforcement and someone calls you with a tip about a deranged individual who has guns, has talked about killing people, and has expressed a desire to perform a school shooting, do you just throw your hands up and say "sorry pal, what do you expect me to do with this??" Or, do you get off your ass, do your job, and go investigate it?
Maybe they get the kid to admit to his plot. Maybe they get the kid admitted for psychiatric help. Maybe they uncover enough information to charge him with something. Maybe it turns up nothing and he goes on to commit the crime anyway. At least a serious effort would have been made to prevent a shooting based on credible information.