Students at St. Joseph's Catholic High School in Barrie were sent home and given the day off on Wednesday after the fire alarm sounded.
Fire officials confirmed the fire had broken out in one of the washrooms.
All the students and staff members at the Cundles Road school safely evacuated the building, and the fire has since been put out.
Due to heavy smoke, the Simcoe Muskoka Catholic District School Board (SMCDSB) decided to dismiss students for the remainder of the day.
Students cannot access the building to retrieve their personal belongings for safety reasons.
The fire department is working to ventilate the building.
Buses were called to the school to bring students home.
Please note the school parking lot is closed at this time and will not be accessible for student pick-up.
SMCDSB will notify families when the parking lot has reopened.
Students who cannot walk or take the bus will be sent to Monsignor Clair School, where staff supervision will be available. Those who can walk will follow their regular route.
It's unclear if the school will be reopened on Thursday.