In California, where I teach, a school will go into a form of lockdown where each classroom is locked, no students can leave their classrooms without an adult chaperone (i.e. bathroom, etc). While this will not help the class that has the person with the gun if that person decides to follow through on their threat (and if the threat was viable), it does minimize/isolate the damage. I believe if the internal lockdown continues for a certain amount of time, an early release is called. We have had several "prank" threats in our area, and this is what has happened. Often, once families get word, they start picking their children up early. There was a HUGE issue when there was a viable threat, but LE asked the school to hold off so they could search the student's home and arrest them off campus. Families were very angry because their children went to school that day. The threat was discovered the night before, and LE wanted the school day to do their investigative work.