Law enforcement officers planned to increase their presence at Charter Oak High School in Covina Friday following an on-campus fight and a threat, officials said.
"We have been made aware of a subject mentioning a threat to take place tomorrow on the campus and it is currently being investigated," Covina police said in a Facebook post on Thursday, saying that they were working with Los Angeles County sheriff's deputies to "ensure the safety" of students and staff.
The post mentioned arrests made after a fight between a student and a non-student on Wednesday.
"This situation has escalated and several posts have circulated on social media," the Police Department said.
According to the San Gabriel Valley Tribune, officials earlier said a 16-year-old student lost consciousness after being placed in a "wrestling-type hold" by a 17-year-old student in a locker room on Wednesday.
The younger teen regained consciousness and was arrested on suspicion of fighting on campus before being cited and released, the Tribune reported. Authorities arrested the older teen on suspicion of assault and had planned to take him to juvenile hall, Regan told the paper.
Charter Oak High School has not released a statement on the case.
"According to school officials, student attendance is very low today and there have been no incidents on campus," police said in a 10:43 a.m. tweet.
KTLA's Alexandria Hernandez contributed to this report.