Toronto police now say a three-alarm fire that broke out at Danforth Collegiate and Technical Institute Wednesday was arson.

Police confirmed Friday that they are conducting a criminal investigation at the school.

Students in a drama class on the main floor first discovered flames in a storage closet at approximately 10:15 a.m. on Wednesday. Heavy smoke could be seen coming from the school, near Danforth Avenue and Donlands Avenue.

All 1,200 students and 160 faculty members evacuated the school safely, and a firefighter was treated at the scene for heat exhaustion.

"By the time [firefighters] got to it, it was very tough to reach, the fire was so intense in there, it was rolling over their heads," said Toronto fire platoon chief Norm Clements.

Students were bussed to other schools to attend classes on Thursday and Friday. The TDSB said students will only have a reduced school day on Monday, starting at 12:25 p.m.

Danforth Tech remains closed for repairs and will not re-open on Monday. The TDSB said school staff will meet on Monday to discuss the next steps.

Chris Broadbent, health and safety manager for the TDSB, said the police investigation is continuing, meaning the board is not yet able to get into the school to assess the monetary costs of the fire with an insurance adjuster.

They're still not certain when students will return, Broadbent said, but student safety is paramount.

"They won't be coming back in here until we ensure its safe," Broadbent said.

Broadbent said the school has extensive smoke damage. Windows were broken by firefighters in order to vent the fire and there is some water damage.

Some students were allowed back into the school Friday to gather belongings the left behind on Wednesday.