Introduction - A cluster of cases of illness occurred at Carignan Middle School in the Ardennes department. An environmental assessment and an epidemiological investigation were conducted to confirm the existence of an outbreak and to evaluate the school’s indoor air quality. Observations - The cases primarily involved students in a 6th-grade class. Ten students were hospitalized. The illness episodes began after a girl fainted in the schoolyard during recess and the fire alarm was accidentally triggered twice in a row. Environmental analyses did not identify any source of toxic exposure. The follow-up study by INERIS concluded that indoor formaldehyde concentrations exceeded toxicological reference values. Discussion - The epidemiological investigation confirmed the occurrence of the outbreak, whose characteristics suggest a psychogenic origin. The unusual events that preceded the series of fainting episodes can be considered triggers and amplifiers of this outbreak. The suggestion of a psychogenic origin is very often poorly received. The results of the epidemiological investigation were not taken into account to expedite the reopening of the school, which remained closed for over a month. (R.A.)