At least 20 people were injured Wednesday morning at Franklin Regional High School in Murrysville, Pa., emergency officials said.
No fatalities have been reported, Westmoreland County emergency management spokesman Dan Stevens told ABC News.
The male assailant, whose identity was not released, is now in custody. He is reportedly a student at the school.
At least four people with “serious” injuries were flown to area hospitals. Most of them sustained stab wounds to their extremities, Stevens said.
It is unclear how many of the injured were students or staff. Two hospitals reported they have nine patients ages 15 to 60 injured in the incident.
“One of the children that pulled the alarm, my daughter said, after the incident happened to clear the school. She reported to me it was a 10th grade student,” parent Tim Graham told KDKA.
The Franklin Regional School District website says, "all elementary schools are cancelled, the middle school and high school students are secure. Additional information will be released as soon as possible. Please keep our campus clear of traffic."