The violence was to have taken place at South Pasadena High School, local police said yesterday in a statement.
Police averted a potential mass shooting after receiving information from school officials, the statement from the South Pasadena Police Department said.
The two adolescents had planned to kill three staff members and as many students as possible, the statement said.
The statement said school officials became aware of "suspicious behaviour" and reached out to police.
One student was arrested without incident while the other put up resistance as police tried to arrest him at his home and he tried to flee, the statement said.