74 people have been killed or injured by guns at American schools this year

On Monday, three children and three staff members were fatally shot at the Covenant School in Nashville, which one expert describes as part of an "astronomical increase" in violence on school campuses in recent years.

There is no universal definition of a school shooting, explains Jillian Peterson, an associate professor of criminology and the president of the Violence Project, a non-profit research center.

"In my research, we focus on school mass shootings, which is a really narrow definition where somebody comes, is heavily armed, fires indiscriminately, and kills four or...