The threat to R.D. White Elementary School in Glendale was made about 9 a.m., the first day of school after the holidays, the Los Angeles Times reported.
Glendale police Sgt. Tom Lorenz said the school's 880 students were evacuated to the parking lot of a nearby grocery store while officers searched "every inch" of the school building.
Schools in Glendale have been under extra police patrols since the Dec. 14 school shooting in Connecticut.
Lorenz said parents of the R.D. White students could pick up their children at the parking lot if they had proper identification.