Riverside County sheriff's investigators said 18-year-old Fernando Hernandez, a student at Canyon Springs High School, left the threatening note for the teacher in a place she would find it. She alerted authorities upon discovering it.
Deputies search the teen's home and found a rifle and ammunition. Investigators said the rifle was legally purchased and secured with a trigger lock. Hernandez was arrested on charges of making terrorist threats.
The threat comes in the wake of a shooting at Sandy Hook Elementary School in Newtown, Conn., which left 20 children and 6 adults dead.
Many students and parents say they were not notified by the school about the incident, and instead found out through social media.
"I actually found out on Facebook," said student Hanah Martin. "That's the reason I found out, I didn't know."
The school district said a phone message was sent out to all parents notifying them about the incident, and a letter would be sent home with students Tuesday.
Students said they are left feeling unnerved, and they just want all the threats and shootings to stop.
"I know school is supposed to be a safe environment, but I'm kind of scared now," said student Nathan Ritchie.
There have been other threats since last Friday's shooting. On Sunday, 24-year-old Kyle Bangayan was arrested after police said he posted threats on Facebook about killing elementary school students. Authorities found several guns and ammunition at his apartment. But the district attorney's office said it will not file charges because there was no specific threat against a school or a person.
There were also two other school threats, one in Diamond Bar and one in San Dimas. In both incidents, no criminal intent was found and no charges were filed.