Two people were shot at Taft High School in Bakersfield, Kern County, about 120km north of Los Angeles, officials said.
Kern County Fire Department said one victim refused treatment at the scene and the other was transported to Kern County Medical Center, Fox News reported.
The victims' ages were not immediately known.
The ABC news reported that the station received phone calls from people inside Taft Union High School who were hiding in closets.
The situation is said to be under control and students were evacuated to a football field.
City manager's assistant Debra Elliott says the shooting was reported shortly after 9 am (local time). She did not provide further details.
Taft is a city west of Bakersfield with a population of about 7,000.
Reports of the latest shooting comes as the country debates gun control after the deadly shooting inside a Connecticut elementary school last month.
Last month in a gruesome incident, a gunman had killed 26 people, including 20 first-graders, at the Sandy Hook Elementary School in Connecticut's Newtown.
