At my school we do live drills twice a year. Have police roaming the hall shooting blanks. Have students screaming as if shot.
Police grab every classroom door and try to open it and look in through glass in door trying to see students. If they can, they come back after the
drill and tell them, "your dead." And they are not being funny about it.
Once last year the principal came on and said, "active shooter, this is not a drill." Every student was in place where they needed to be in just
a few seconds. You could not have asked for better. Turns out a man was walking across the school parking lot with a rifle. He had just bought a
pellet gun at a garage sale one block away. I have to give it to our police, they were here in 90 seconds and had him surrounded with guns drawn.
No waiting for SWAT team to arrive.