Someone called 911 at 9:16 a.m. and said bombs in four separate bags would be set off on the Mast Boulevard campus 20 minutes later, according to the San Diego County Sheriff's Department.
"It sounded like a (boy) saying that he was tired of being bullied and (that) he had put backpacks around the school that were going to explode, basically,'' sheriff's spokeswoman Jan Caldwell. "If this is a hoax, it's a hoax. If not, we have our units here that will take care of it.''
The Grossmont Union High School District reported via Twitter that the lockdown was triggered by an "unsubstantiated threat that law enforcement is investigating.''
The lockdown was lifted around 1 p.m., though the school remained in a lower-level precautionary status known as "secure campus,'' according to school officials. Students were seen leaving campus soon after the lockdown was lifted.
It was not immediately clear if the suspicious packs turned out to have anything hazardous inside them.