The Los Angeles Police Department said the threat was made on Snapchat Sunday and has been circulating all over social media.
Investigators are still trying to figure out where the threat came from and who made it.
Authorities also have not said whether they believe the threat is credible or not.
Tanesha Garia said her son was frightened by the threat.
"He was up all late last night trying to figure out what's going on, should he come to school, wondering about his friends," she said.
One student said most classes were half empty. The principal announced late morning that the school was safe, and that everyone should be back in school on Tuesday.