Police spokesman Mark Fulghum says it was a small mortar-type firework that had been wrapped in tape. The boy forgot he had it in his backpack and it was found by a school security guard.
The school was evacuated about 9:30 a.m. Wednesday and students waited at the performing arts center across the street until police determined the building were secure.
District spokesman Dan Voelpel said the student has been expelled.
Century-old Stadium High has about 2,000 students and is known for its castle-like architecture.
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