If you are in crisis, you cannot be effective. Let them know that everyone reacts differently, on their own schedule, and all feelings are acceptable. Let them know that asking for help or seeing a counselor is okay. Encourage discussion of events. This allows students to express some of the emotions.
Discussion also dispels rumors and misinformation.
Encourage support systems. Groups become vital to re-establish a sense of safety and belonging. Grieving and trauma recovery are processes which can't be rushed. Remember that one of the hallmarks of trauma is difficulty concentrating and processing information. Do not expect students to perform well in the weeks immediately following the traumatic event.