There are more attempts at mass murder by terrorist and other people who want to kill as many people as possible in schools, shopping malls, and events that attract large numbers of people. Schools must be prepared for active shooters because it is a very real possibility that a shooter could enter any school.
I am 100% for school drills like this one (demonstrating what an active shooter situation sounds like) so our children are aware, in this day and age where children are sometimes not safe in schools, However an officer accidently discharging a live round is pure negligence on the part of the officer and he should be held accountable for this.
Drills at schools preparing students for terrorism or violence are important. However, live ammunition should never be used at these drills. Although the chance of an accident is tiny, any risk to students during a drill is unacceptable. Effective drills can be conducted with the use of realistic looking weapons that do not have any live ammunition.
The active shooter drill is unfortunately something kids need to practice. However, there's not much reason to have live ammunition at a school. If it's a military school and students are being trained to handle and use live ammunition, it's still something that should be done under very controlled conditions. In the active shooter drill, there's definitely no reason to have live ammunition at a public middle school.