Investigators recovered a shotgun they believe was used in the shooting at Taft High School in Taft, said Ray Pruitt of the Kern County Sheriff's Office.
Pruitt didn't know whether there was any relationship or connection between the shooter and the victim, he told CNN.
The victim was airlifted to a hospital in Bakersfield, Pruitt said.
The shooting occurred in the science building, but Pruitt said he didn't know if it took place in a classroom or the hallway.
Authorities were still in the process of searching and securing the school.
"There are still some students on campus," Pruitt said late Thursday morning. "We're still searching from building to building because we want to make sure we don't have suspects outstanding."