Officials said Friday that Wednesday's fire at Danforth Collegiate and Technical Institute is now considered a "criminal matter."
The fire forced the evacuation of the high school, which is located near Danforth and Greenwood avenues.
No one was hurt but the school suffered smoke and water damage. Many of the classrooms have broken glass on the floor.
The Toronto District School Board says students at the east-end high school will not be able to return to the school on Monday. Instead, students will be temporarily relocated to other schools until the fire damage is repaired.
The TDSB says it will provide an update on the situation on Monday afternoon.