Administrators at NOVA Academy Early College High School in Santa Ana said they plan to have armed guards patrol the campus at least through mid-April, the Los Angeles Times reported.
The former girlfriend reported the email to school officials Monday and they got in touch with police. The 16-year-old boy was taken to juvenile hall.
The boy in his message allegedly identified students he said would help him with his campaign. He said he was acting because he was sick of being "bullied" and did not list the former girlfriend as a target.
Santa Ana Police Cpl. Anthony Bertagna said a search of the teen's home found no weapons.
A restraining order has been issued ordering the teen to keep away from the school and from his ex-girlfriend.