"How about an emergency alarm that warns every classroom that there is an intruder in the building?"

I asked at my sons school. I was assured that they have a separate alarm sound - apart from the fire alarm - that tells teachers that there is a dangerous situation going on and to take all precautions to protect the students. Let's hope that all the other school systems have something similar now. I think concerned parents should ask their school if this system is in place at your child's school.