The people of the Pine Ridge Indian Reservation are no strangers to hardship or to the risk of lives being cut short. But a string of seven suicides by adolescents in recent months has shaken this impoverished community and sent school and tribal leaders on an urgent mission to stop the deaths.

On Dec. 12, a 14-year-old boy hanged himself at his home on the reservation, a sprawling expanse of badlands on the South Dakota-Nebraska border. On Christmas Day, a 15-year-old girl was found dead, followed weeks later by a high school cheerleader. Two more young people took their lives in February and two more in March, along with several more attempts.

Students in the reservation's high school and middle school grades have been posting Facebook messages wondering who might be next, with some even...