Stones to Stop a Shooter?

In an interesting twist a principal in Pennsylvania has “armed” his classrooms with stones. The premise being the school could stone the shooter?

https://www.washingtonpost.com/news/grade-point/wp/2018/03/23/this-school-districts-plan-to-stop-shooters-a-bucket-of-rocks-for-students-to-throw-at-them/?utm_term=.5c4c46f0b74d