Columbine High School in Colorado was placed on lockdown amid threats that explosive devices had been placed in the building.
An anonymous caller also claimed he was hiding outside the building with a gun, police said.
Parents of pupils at the school, where 13 people died in a shooting in 1999, were told by local police to stay away as the incident unfolded on Thursday.
Some 24 schools in the area were placed on lockdown, school officials in Jefferson County said.
Mike Taplin, spokesman for the local sheriff’s office, said someone called to make the threats at 9.20am (4.20pm GMT). A large police operation followed involving bomb squad officers and helicopters.
He said: “Today we received a phone call with a threat made of multiple explosive devices inside the high school. The person also claimed to be hiding outside the school with a gun.
“The perimeter had been searched and secured. No one was found.”
"Right now everyone is safe and we're asking everyone to stay out of the area," he added.
Schools in Columbine, Chatfield and Dakota Ridge areas were affected, Jeffco Public Schools tweeted.
Two ex-students shot dead 12 pupils and one teacher and injured many more at Columbine High School before killing themselves in 1999.
