Students, Mindful of Columbine, Break Silence to Report Threats
Psychologists from the Secret Service, with whom Mr. Modzeleski works, found that in almost three-quarters of 37 school shootings since 1974, the assailant told someone in advance about his plan, almost always another student.

That means many school shootings could be averted if students shared information with teachers, administrators or parents, said Marisa Reddy, a director of the Secret Service's Safe School Initiative.

http://www.nytimes.com/2001/02/10/national/10RAMP.html?pagewanted=all