What I think that can be done to protect the lives of students and teachers from an active shooter is a new way to stop a door. The doors today are strong but can probably be opened by using a shotgun on the handle or maybe even with a lockpick. The idea that I have is to have a wall that goes parallel to the door that has an amount of space in between that a desk can fit perfectly. There is almost no way that the door will be able to be opened from the outside. Below I have attached a picture to help you understand.