As a retired teacher in an elementary school, here is how it was. I remember Columbine. We had guest readers in the room reading to the kids and they came and told me the info,
After that, the principal met with the staff as there was a new law that all schools had to have lock down drills at least six times a year.
We were to examine our classrooms and look for a place to put the children where bullet trajectories could not reach the children through a window. We were to not bunch the kids together as bullets can go through them like butter. We were to try to find a wall that bullets could not penetrate, My windows were up above the radiators so I had to climb up and pull the shades .
I do not recall what I told the children as absolute silence had to be expected as well.
We had windows in the doors of our classrooms, but they were all covered now so no one could look in.
We got new schools built and they were all built to the National Standards for school buildings designed to deal with school shooters. Isn't that special!
I retired before the new school I would have worked in was completed so I do not know what the design entails except controlled access.
I was in the generation of kids who hid under their desks for nuclear attacks. We have a sister city with a city in Russia. The people there said they were terrified of us because they learned how horrible we were.
The days I was out on the playground and the lockdown was announced were always fun. No place to hide when out there, The teachers at Sandy Hook had just had a lock down practice, I believe.