One afternoon in the spring of 2006, for reasons unknown to those who knew him, Mitchell Henderson, a seventh grader from Rochester, Minn., took a .22-caliber rifle down from a shelf in his parents’ bedroom closet and shot himself in the head. The next morning, Mitchell’s school assembled in the gym to begin mourning. His classmates created a virtual memorial on MySpace and garlanded it with remembrances. One wrote that Mitchell was “an hero to take that shot, to leave us all behind. God do we wish we could take it back. . . . ” Someone e-mailed a clipping...