At 2:35 p.m. LAPD Officer Bruce Borihanh said that it was "an explosive device" that was rendered safe.
Nearly 2,000 students and staff members spent much of the school day in the gymnasium and on the athletic field until the bomb squad determine the device was safe.
Students left the campus from the 42nd Street side at 3 p.m., the normal time.
School is in session throughout the year at Manual Arts.