This. Like I've said several times... we have officers in our schools. They've stopped things from happening before it was too late. While they are there, our patrol officers go to each school several times a day to do a walk through. It's helpful.
The only issue with that is in many places the funding for police has been cut (Camden, NJ, for example, had to drastically reduce its police force), so in a lot of cases they don't have the manpower to do it.