This report outlines the various stages of investigation and management of a cluster of nonspecific symptoms that occurred at a high school in Pas-de-Calais in March 2010, an event that was both brief and limited in scope. It illustrates the value of early, coordinated on-site intervention, following the steps recommended by the French Institute for Public Health Surveillance’s guide, “Diagnosis and Management of Unexplained Mass Illnesses,” published in 2010. The rapid response to the alert and the establishment of an interdisciplinary coordination team made it possible to coordinate management decisions with investigations, ensure consistent communication, and formulate appropriate explanatory hypotheses. Furthermore, the rapid on-site response made it possible to avoid launching environmental monitoring campaigns initially requested by the principal and his team. No benefit was expected from these campaigns in explaining the symptoms or identifying a source of pollution after the fact. Indeed, experience shows that such campaigns only identify causes if there are clear hypotheses, established prior to measurement, targeting specific substances suspected of still being present. Finally, the clinical expertise of the teams from the Mobile Emergency and Resuscitation Service (SMUR) and the Emergency Department at the Boulogne-sur-Mer Hospital, which highlighted a psychosocial spread of this event, most likely contributed to the rapid resolution of the episode. (R.A.)