Shortly after 2 pm on Monday afternoon, it started to burn at Hagagymnasiet in Borlänge.
According to the police, it must have been a small fire in a trash can.
A room must have been filled with smoke and the school was evacuated.
The fire could be extinguished quickly by staff at the school and the rescue service focused on ventilating the smoke.
It is unclear if any person was injured in the fire.
The police suspect that the fire has been started and a report of a suspected arson has been made.
There are no suspects at this time.