If the stressful situation would be anything like Columbine, Virginia Tech, or the theater shooting, then students' stress comes in many forms. Emotionally, the students need to talk to someone who can help them understand the anguish they are feeling. Allowing them to vent their emotions so that they do not well up enables the students to externalize rather than internalize their anger, frustration, and sadness.
In addition, the parental involvement emphasizes to the students that their lives will go on, and the adults in their lives are there for them. Again, letting the students talk and just listening is a stress reliever in itself. The adult reaction to the unimagineable situation will determine the relief that the teenagers experience.
Structure and returning to as much normalcy as possible allows the student to grieve but within a safe place. Everyone involved in the situation must go through the grieving process. If not, then many times he will become "stuck" in one of the stages of grief...particularly anger.
Helping the student to realize that there are no easy answers to his questions of "why" or "what" or "if." No one can fix these situations, but the counseling and listening can help everyone to open lines of communication.