Will work for many crazies out there wanting to shoot up a school. However, the Florida shooter shot kids as they were going outside so not much use for it for that tragedy. However, if I were a Principal, I’d consider getting them. Why not?
They are kept behind the door, ready for use.
In about 10 seconds, some problem kid will figure that he can use the device to keep out teachers, school officials, and other students, for whatever nefarious plan he might be dreaming up.