The station says the suspect, a sophomore at Nova Academy Charter School in Santa Ana, Calif., wrote an email to his ex-girlfriend saying he had a "hit list" of students he wanted to shoot, according to police.
The boy reportedly told the girl, who's also a student at the school, that he was going to get help from others, and that the plan for the attack was the result of years of bullying, which allegedly began when he was in middle school.
The emails were brought to the attention of school officials Monday night.
"Last night two students went to the principal, provided [her] with some documentation of what was going on. She immediately did the right thing, called the police department," said Cpl. Anthony Bertagna of the Santa Ana Police Dept. He said "we came out, investigated, they interviewed the male student and based on the information that we got from the interview and the evidence the officers obtained, they arrested him."
Investigators say they didn't recover any weapons on the student, CBS Los Angeles reported. The suspect was being held at a juvenile detention center.
The campus was placed on lockdown Tuesday. Authorities say they're still looking into when the alleged attack was to take place.
Police said they will continue to protect students until the school hires its own security.
School Principal Renee Lancaster said, "Our community is about taking care of our families, and that's what we pride ourselves in doing, and in doing the right thing. And we want this student to get some help, if needed, and we care about him, just like we care about every student here."