I hate to sound like a sheeple, but there is no 'real' planning for this case.
There is never really a plan but there are things to help deter it from happening. Many businesses in my neck of the woods hire LE for their security year round. It being the holidays if you go down to the mall right now there is a mobile command station and about 8 sheriff units parked out front. Its like that every year. We have an officer in every school and the larger schools have 2. Thats not a security for hire either Safe School is a division of the sheriff's office. Shootins will still happen though, the last few shooting we have had in public places were personal vendetta or gang related drive by against specific people.