I'm an :chomsky-yes-honey: so I was last in public school many years ago. We would have them maybe twice a year, they would make announcements using code phrases and the teachers were trained to know what they meant (either shooter outside the building or inside the building).

EDIT: Now that I think of it, I went to public school in the era that was actually the immediate fallout of Columbine, so I presume this was a fairly novel practice. I guarantee it's way more dystopian now.