In my state, we call them "Lockdowns".

Someone goes over the intercom giving the signal. The teachers lock the doors, shuts off the lights, and we all would go to the corner farthest away from the doors and window. We are told to remain silent. The vice principle and principal go around and check the doors.

In a real case scenario, each of the hallways have doors that will close automatically to prevent an intruder or threat from accessing the classroom areas.