thwarted attacks

kcab was right:

[A] December 2001 article in the Boston Globe reported that since the April 1999 Columbine tragedy, 12 U.S. school rampage shootings have been discovered and thwarted before they came to fruition.

Moore, Mark H., Petrie, Carol V., Braga, Anthony A., and McLaughlin, Brenda L., ed. Deadly Lessons: Understanding Lethal School Violence. Washington D.C.: National Academies Press, 2003. (296.)