Most schools practice active shooter drills by “calling” the drill while students are already in class. When the drills are called, the teacher shuts the door (that typically is already locked from the outside) and the students sit in the corner away from the door. All are required to be quiet until someone comes and checks the door. Once all doors are checked, someone from administration comes over the loudspeaker and “clears” the drill. If a school is responsible, they will require a two-person verification at this point, meaning two people must come over to “clear” the drill from two different locations in case one of them is being held hostage.

Student Safety in Active Shooter Situation- Part 1, by P.N. – SurvivalBlog.com