The new official plan is called ALICE, which is based on the miserable failure of hide in place at Virginia Tech. If the intruder is a distance from you, you evacuate. If he is next door, you want to keep him from coming into your room. Of course you lock the door. But if the door has a window (and most schoolroom doors do) you want him to be unable to open it even if he shoots through the glass and tries to unlock it. The goal is to be able to keep the door closed and keep the bad guy out. Because classroom doors open out, a simple barricade won’t help. A length of rope tied to the doorknob and held by several strong people off to the side might do it or several turns of rope around the arms of the door closing mechanism.
The last resort—when the bad guy has still managed to get in in spite of your best efforts, is to throw stuff, including oneself, at him. Because these event have shown us that he will kill as many as he can until he is stopped.
“throw the book at him”
I used to carry Large ALICE in ancient history in the Army.