According to this article, commentary in the Canadian Medical Association Journal has indicated that "debriefing" in response to crisis incidents is not suppoted as an intervention. The commentary specifically notes that there has been no research to document that debriefing prevents the development of PTSD or any other disorders following a crisis in schools. from the article:
Recent systematic reviews indicate that psychological debriefing of adults does not prevent post-traumatic stress disorder and it may even increase the risk of this disorder. While there is little research on the effectiveness and safety of these interventions in schools, "the evidence clearly points to the ineffectiveness of these interventions in preventing post-traumatic stress disorder or any other psychiatric disorder in adults," write Magdalena Szumilas of the Sun Life Financial Chair in Adolescent Mental Health Team, Dalhousie University and coauthors.
The article goes on to suggest that providing "psychological first aid," which is defined as creating an environment od safety, calmness, and both self- and community-efficacy, shows more promise as an effective intervention technique. In addition, providing cognitive-behavioral support for students who continue to exhibit symptoms of distress in the weeks following a critical incident is also considered (due to empirical findings) to be the more effective procedure.
This is certainly consistent with other data I've seen, which basically holds that while debriefing may be beneficial for some, it may actually exacerbate the traumatic impact in others - and there is no way to predict who is who. More generally, the recommendations regarding psychological first aid and cognitive-behavioral therapy appear consistent with the general process of therapy for an individual with trauma-based symptoms. You simply don't begin therapy with a traumatized client by discussing the traumatic event. A sense of saftey and security must first be established, where the client can feel as if the therapeutic environment will protect the person, and not further traumatize them, is of paramount important in many cases. In addition, before discussing the traumatic event with a client in any manner that might lead to an increase in anxious symptomatology, I frequently work with a client on developing skills and techniques necessary for self-soothing, in order to develop a client's sense of efficacy. In fact, that is one of the primary goals of therapy related to trauma: re-establishing a sense of control over one's symptoms, both physically and mentally. Jump into discussing the actual event too quickly, and you risk exacerbating a client's sense of helplessness. People need to be educated on this, of course, so that they understand the reasoning behind any delay in "getting to the issue, but once this is explained, most people understand.