Classes at Florence-Carlton School were cancelled Thursday after an early morning fire in the boy's locker room.
Florence Fire District Board Chair Clint Roberts said firefighters were dispatched to the school at 7:07 a.m.
At 8 a.m., crews were still at the scene.
Roberts said a janitor had knocked down the fire by the time firefighters from both Florence and Missoula Rural arrived on the scene, but there was heavy smoke inside the school.
The cause is still being investigated, Roberts said.
A press release from the Ravalli County sheriff's office said it appeared to be an electrical fire.
The Ravalli Republic will provide more information as it becomes available.