Emergency crews responded to a fire at F.E. Madill Secondary School on the morning of April 7.
Huron County Ontario Provincial Police, along with Huron County EMS and the North Huron Fire Department, were called to the scene shortly after 8 a.m.
The school was evacuated as a precaution, and fire crews were able to successfully contain and extinguish the fire.
One person sustained life-threatening injuries and was transported to a local hospital.
Police say there will be a continued emergency presence in the area and are asking members of the public to avoid the scene.
School-related inquiries are being directed to the Avon Maitland District School Board. The school will be closed for the rest of the day and that students have been safely relocated to Maitland River Elementary School.
Investigators are also asking anyone who may have video footage of the incident not to share it online, but instead provide it directly to police.
The investigation remains ongoing. Anyone with information is asked to contact the Huron County OPP at 1-888-310-1122 or Crime Stoppers anonymously.