Officials said they received a call about a man with a gun on the high school's campus.
The former student, who transferred in recent months, took out an SKS-style carbine and fired about 10 shots, video of the incident appears to show, according to Los Angeles County Sheriff Jim McDonnell.
Investigators are still trying to determine a specific motive.
"The Los Angeles Unified School District police department reported at least 160 such threats in the nine days after the Parkland massacre, and a tip from a school resource officer led Los Angeles County sheriff's deputies to arrest a 17-year-old student at El Camino High School in Whittier that month who is accused of threatening to "shoot up" the school after a teacher took away his earphones", The Los Angeles Time wrote. The sheriff's office said a boy had been shot in the arm at the school and was in stable condition at a local hospital.
Authorities say the suspect ran off after the shooting and dumped the rifle along the way before he was caught in a shopping center. "Alexander, turn around, turn around, there's an active shooter.' And I didn't really have time to react to that because nearly immediately after that I heard one single gunshot very loud and very close by".
Agents with the Department of Alcohol, Tobacco, Firearms and Explosives (ATF) were heading to the scene, the agency said.
"About a minute later, I get a call from them saying there were gunshots" and they asked to be picked up, Munoz said.
The shooting happened at Highland High School in the high desert of Antelope Valley north of Los Angeles.
Sheriff's deputies also headed to a second school, Manzanita Elementary School, a few miles away after receiving reports around 7:30 a.m. that people thought they heard gunfire, Boese said. "This is ridiculous - there's too many school shootings", said parent Tracy Reed, whose son texted her about the shooting at his school. She told the Los Angeles Times that he was arrested in a nearby parking lot.