Sheriff's officials in Walnut said explosive detectives were on scene at the school located at 1100 N. Grand Avenue after a suspicious package was reported shortly before 7 a.m.
Authorities said the campus quad was taped off, and a school official said the music building was evacuated.
A message from the school's Twitter account warned students to stay away from the school theater, music building, campus caf and other areas, and said some classes were canceled.
EMERGENCY ALERT: Suspicious package. Stay away from Theater, Music, 1A, 1B, 1C, 6, Campus Cafe. Classes before 9:30 in those bldgs canceled.— Mt. SAC (@MtSAC) December 1, 2015
The school later tweeted that investigators cleared the campus and that the object was not a threat.
Emergency personnel have given the ALL CLEAR. Suspicious package is NOT a threat. You may return to your normal activities.— Mt. SAC (@MtSAC) December 1, 2015