Police arrested a student who they said brought a small improvised explosive device to Eckstein Middle School in north Seattle Thursday morning.
Staff at the school found the components for the device in a search of the student's backpack, and called police, the Seattle Police Department said.
The small device was similar to a large firework and was not fully assembled, police said.
"Unless it was close to somebody, it would have only caused minor damage," said Sgt. Verner O'Quinn in a news release.
Police booked the student into the Youth Service Center on suspicion of possession of an explosive.