One official said some students suffered "significant injuries"
As many as 20 students have been injured in a mass stabbing at a high school in the US state of Pennsylvania, authorities have said.
One suspect believed to be a student is in custody at Franklin Regional High School in a suburb of Pittsburgh, a local affiliate of ABC News reports.
The wounded, at least four with significant injuries, range in age from 14-17 and have been taken to hospital.
The school was locked down as police searched the premises.
"A critical incident has occurred at the high school. All elementary schools are cancelled, the middle school and high school students are secure," school officials wrote in a bulletin following the incident on Wednesday morning.
Four students are seriously injured, while other students were said to have suffered scrapes and cuts in the mayhem that ensued following the attack, emergency management spokesman Dan Stevens told US media.
None of the knife injuries, said to be slashes, were believed to be life-threatening, Mr Stevens added.
Several people - including one adult - were flown to local hospitals with superficial to quite serious injuries, the Pittsburgh Post Gazette reported.
Television images of the scene showed students standing in groups outside the school with ambulances nearby.
A parent of a student told CBS television his "frantic" daughter called him to say a friend of hers had been stabbed at the school in Murrysville.
"I would never expect this to happen," he said, adding that he had believed the community to be safe.
The suspect, a male student, has been taken into custody for questioning.