Fire crews were dispatched to the school shortly before 8:30 a.m. when someone reported the classroom was filled with smoke.
No one was using the classroom at the time, Snohomish School District officials said. The portable is used for occupational therapy.
Firefighters determined the heating unit was the most probable source, Snohomish County Fire District 7 spokeswoman Autumn Waite said. They cut into the side of the building to gain access and pulled down part of the ceiling to confirm there was no active fire.
Students and staff were allowed to return to the other classrooms around 9:30 a.m.
Damage was estimated at $10,000.
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