An anonymous caller phoned the R.D. White Elementary School in Glendale at around 8:30 a.m. and said there was a bomb at the campus, police Sgt. Tom Lorenz said.
The school's 880 students were evacuated to a supermarket parking lot about a block away and parents were allowed to take them home for the day, Lorenz said.
Klara Esposito, who lives a block from the campus, received a call from the school and came to pick up her two children, 10-year-old Tony and 7-year-old Daniel.
"I mean after Connecticut, it was just too scary," she said, referring to last month's massacre by a gunman at Sandy Hook Elementary School in Newtown, Conn. "I just ran here like a maniac."
"A few teachers told me it was all just a drill," her son Tony said. "And I was thinking if it was just a drill, why were there so many policemen and firemen and helicopters? I'm really scared."
Glendale is a foothill suburb of Los Angeles.