But acting on information provided by officials at South Pasadena High School, authorities were able to foil the plan, police said.
"This is a prime example of school officials recognizing suspicious behavior. It was this information that helped prevent a horrific tragedy," Sgt. Brian Solinsky said.
Authorities have scheduled a news conference Tuesday to offer more details.
One student was arrested at his home Monday without incident. The other, police said, tried to run away -- but didn't succeed.
The South Pasadena Unified School District sent a letter to parents, addressing the alleged plot.
"The police have the situation under control and there is currently no threat to students or employees," the letter said.
The district will make counselors available to students and employees when the new school year begins, CNN affiliate KTLA reported.
"School starts on Thursday, and I'm pretty sure a lot of people will be talking about it," student Owen Carlson told the station.