The boy was arrested Friday after police responded to a call of a threat to shoot staff members and fellow students at Ánimo Mae Jemison Charter Middle School in South Los Angeles.
Several students notified teachers after overhearing a threat that a shooting would take place at the campus on Friday, Sheriff Alex Villanueva said at a news conference.
"There was a rumor about this kid that was going to kill people," one student said.
Police executed a search warrant at the teen's home and seized an AR-15 rifle, a large cache of ammunition, "a list of intended victims and a drawing of the school layout."
A 19-year-old relative of the teen was also arrested at the home where the search warrant was served. He was arrested for possession of an unregistered firearm and is being held on $35,000 bail.
The firearm was unregistered but does have a serial number, investigators said, adding that it did not appear to be a so-called ghost gun.
The suspect's name was not released because he is a minor. Police are investigating a motive.
On the heels of the country's most recent school shooting at Saugus High School in Santa Clarita, the Los Angeles County Sheriff said more than 30 threats on other schools were reported.
Of those threats made, another led to an arrest.
"With all incidents like these, we ask all Los Angeles County residents that if they see or hear something, to say something," the sheriff said. "In this case, the fact that people stepped forward and said what they had heard led us to be able to prevent a tragedy today."