Officials said the fire started in the roof of the school's main building while workers were grinding metal around 3 p.m. Tuesday and spread quickly.
"They were grinding on some metal up there. The spark got into the roof structure, started the roof on fire," said Assistant Fire Chief Joe Johnston.
Witnesses said flames were seen shooting through the roof of the building at 9001 Airport Road.
Huge plumes of smoke filled the sky above the school as firefighters tried to douse the flames from above. A KOMO reporter on the scene on Tuesday evening said the thick smoke, which was being whipped around by heavy wind, was making it difficult to talk or breathe.
Even with firefighters attacking the blaze from several different angles, raging flames were seen scorching much of the roof.
The structural instability of the charred building forced crews to fight defensively from the outside of the building.
Firefighters said combustible material inside the school was fueling the stubborn fire. The building was undergoing an extensive remodeling project.
Students had already left for the day when the fire began. One firefighter was taken to a hospital for evaluation after falling while moving equipment.
Damage to the building is said to be extensive. Students said the section of the school that suffered the most damage was housing the most expensive equipment for programs in the medical, dental and technical field.
"It's a lot of stuff - lab stuff, computers. It's a lot of stuff gone," said student Marissa Timmerman.
Timmerman watched her school go up in flames, horrified with only a few weeks left until graduation.
"It's crazy. I don't know what's going to happen today or tomorrow or if we're going to be able to come back for the rest of the year," she said.
Sno-Isle Tech Skills Center is a public school offering technical training for area high school students. School officials have cancelled classes for Wednesday.