Saunders Secondary School, in London, Ont.'s southwest end was evacuated after reports of a strange odour in the building.
Emergency responders were called to the high school shortly after the noon hour on Wednesday for reports of a foul smell.
Multiple fire trucks and ambulances could be seen outside the school and students were gathered in a nearby parking lot.
Some staff and students inside the school were reportedly suffering from throat irritation.
Nine students were assessed on scene by paramedics.
London Fire Department Platoon Chief Shawn Fitzgerald says a dust collection system in the process of being dismantled for repairs was the cause of the odour.
Crews were ventilating the building before allowing everyone back shortly before 2 p.m. and clearing the scene.
