A second student has been confirmed dead following a shooting at a high school near Los Angeles, the county sheriff has said.

The deaths were confirmed after a boy opened fire on his 16th birthday at Saugus High School in Santa Clarita, 65km north of Los Angeles.

"I'm saddened to report that we have confirmed a total of two fatalities this morning. One female and one male," tweeted Los Angeles County Sheriff Alex Villanueva.

The suspect was in custody and taken to a hospital for treatment, while at least three others were injured in the shooting.

The shooting bore the hallmarks of being another mass shooting in the United States, where repeated outbursts of gun violence in recent years have intensified the debate about gun control and the constitutional right for citizens to keep and bear arms.

Video from local NBC television showed a line of students walking away from the school and a row of police and fire department vehicles parked out front.

Saugus High School and all schools in the William S Hart district were placed on lockdown while authorities flooded the area.

Some 2,300 students attend the high school, which is made up of more than a dozen buildings.

KTLA video, broadcast on CNN, showed one woman being loaded into an ambulance.

"We are doing everything we possibly can to bring this person to justice," Los Angeles County Sheriff's Sergeant Bob Boese told CNN.

Some of the wounded were being treated in a grassy area on the school's campus, the Los Angeles Times reported.