Stadium High School was cleared about 9:30 a.m. and students waited at the performing arts center across the street until police determined the buildings were secure.
Tacoma police spokesman Mark Fulghum said it was a small mortar-type firework that had been wrapped in tape, according to KIRO. The station reported that authorities said the boy forgot he had it in his backpack and it was found by a school security guard.
The school district said on its Facebook page the student was arrested. District spokesman Dan Voelpel said the student was also expelled.
Classes did resume.