The Montebello Police Department announced on Twitter just after 3:30 p.m. that it was aware of the unspecified threat and was assessing its credibility.
"The threat is being investigated and there is a large police presence at the school for the safety of the children," a tweet said.
By 6 p.m., the police department tweeted students and the school, located at 2100 W. Cleveland Ave., were safe and an investigation is ongoing.
Further details were not immediately released.