Oh. My. God. Via Talking Points Memo:
Staff members of an elementary school [in Murfeesboro, Tenn.,] staged a fictitious gun attack on students during a class trip, telling them it was not a drill as the children cried and hid under tables.
...
During the last night of the trip, staff members convinced the 69 students that there was a gunman on the loose. They were told to lie on the floor or hide underneath tables and stay quiet. A teacher, disguised in a hooded sweat shirt, even pulled on a locked door.
More here.