At least a dozen emergency vehicles converged on Kelvin High School on Wednesday afternoon, after smoke started billowing up from a basement crawl space.
At about 3 p.m., Kelvin principal Jim Brown said, construction workers were renovating the school's cafeteria when they noticed smoke smouldering from a section of floor they were cutting.
Employees pulled the fire alarm and left the building, which was mostly empty after summer-school classes let out in the morning. Brown said no one was injured.
It wasn't yet clear exactly what caused the smoke, Brown said, though it may have been triggered by something surrounding the construction efforts. None of the workers saw flames.
By 4:45 p.m., fire crews had the possible fire under control and were venting gusts of smoke out from the building and over Kingsway Avenue.
Winnipeg School Division officials announced that summer school classes would be cancelled on Thursday while crews assess the damage and address the situation.
The rest of the high school was not affected in the incident.