Nobody is reported to have been in the school when the fire started, but the building itself may be completely destroyed.
"It's highly uncertain whether the building will be able to be saved," Mats Streer of the rescue services tells news agency TT.
The school for adult education lies on the outskirts of Malmö, and there is a risk that the fire will spread to other buildings, if the firefighters cannot get it under control.
The police say they cannot yet rule out the possibility that arson lies behind the flames.
The school only recently went up, after a building at the school was totally destroyed in another fire in October of last year. Police suspected arson, but nobody was ever found guilty.