The Los Angeles County Sheriff's Department said the threat was received around 10 p.m. Tuesday but could not confirm which social media platform the threat was posted on.
The Bellflower Unified School District posted a statement to Facebook, saying the threat was made against the safety of students and staff at the school.
Details surrounding the origin of the threat were not immediately known.
Authorities said classes are still in session Wednesday with extra security on campus.
Bellflower High School principal Michael Lundgren said the school takes every threat seriously and is working with the sheriff's department in this case.
The district stated that the sheriff's department has committed resources to the school for Wednesday and the remainder of the week due to the threat.