I had a long conversation with a relative of mine the other day. She's an elementary school teacher in a rough district in another state. We started talking about what she could do, within the rules of her district, to protect her children.
First, she decided to place some useful things in her classroom. These are all common objects that absolutely no one would think twice about, not even the kids.
1) There's a baseball bat leaning in the corner near the door.
2) Heavy three-hole punch located conveniently on the corner of her desk.
3) Grownup scissors inside her desk drawer.
Her classroom door has a window in it. She has a stack of black construction paper and some tape right next to the door. If they go into lockdown, she will cover that window after she locks the door.
The door swings outward, so no use getting a rubber door stop. However, she has a paracord bracelet on a high shelf, along with a plan for lashing the doorknob to her heavy desk in case the intruder has a master key to the school.
One wall of the classroom has knee-to-ceiling windows. She has visualized breaking one of those windows with a chair, and has a heavy rug from the classroom that she could throw over the opening to keep anyone from getting cut as they climb out, if they need to flee for any reason.
The kids all know that if the lockdown alarm sounds, the "class monitor" (special kid for the day) needs to pull the big curtains closed while everyone else gets out of sight in the safest corner of the room.
This isn't the same thing as if she had a gun and was prepared to use it, but it's a whole lot better than, "If the alarm sounds, just hide."
My personal website: Cornered Cat