Internet rumors of Friday the 13th violence at a high school near Lancaster caused hundreds of students to skip school or leave early with worried parents, said school and law enforcement officials.
Also, 10 Los Angeles County sheriff's deputies were sent to the school Friday morning as a precaution.
"This was rumor, upon rumor, upon rumor," said Mark Bryant, principal of 3,400-student Quartz Hill High School.
The rumor involved two former students, ages 15 and 17, who were arrested Dec. 15 after allegedly threatening a Columbine-like attack at the school on Valentine's Day.
The hoax suggested the boys had been released and would plant a pipe bomb on campus Friday. But the teens, whose names were withheld because of their ages, remain in custody at Sylmar Juvenile Detention Center.
By early afternoon, Bryant estimated that as much as a quarter of the student body had gotten an early start on the three-day weekend, as schools will be closed Monday for Martin Luther King Jr. Day.
Lt. Wallace Fullerton of the Los Angeles County Sheriff's Department's Lancaster station said the rumors began to heat up Thursday and were spread by e-mail and cellphone text messages.
Sheriff's deputies conducted a sweep at the campus Friday morning but found nothing.
"We've found that the information has no credibility whatsoever," Fullerton said.
The two boys who threatened the school pleaded not guilty last month to charges of conspiracy to commit murder. Bryant said the Antelope Valley Union High School District had brought in its own extra security earlier in the week to reassure students and parents.
Sheriff's Sgt. Darrel Brown, who oversees the station's campus safety officers, believes the episode may have been a ploy to stretch out the long weekend.
The only action at the Quartz Hill campus Friday was when someone placed a firecracker in a trash can as a prank.
"I was standing 20 feet away when that happened," Brown said. "At first, there was a little bit of a scream, but then [the students] just laughed it off."