One victim was an adult, authorities said, but none of their names was being released.
Dr. Chris Kaufman, the trauma director at Forbes Regional Medical Center, the closest hospital, said two victims were in surgery and one was awaiting surgery. All three were all stabbed in the torso, abdomen, chest or back, which he called "significant injuries."
Seven teens and one adult were listed in serious condition at Forbes Hospital, West Penn Allegheny Health System spokeswoman Jennifer Davis said. They ranged in age from 15 to 60, and some were in surgery, she said. A ninth victim, a 15-year-old girl, was in good condition at Allegheny Hospital, Davis said.
Twelve of the victims were sent to four hospitals in the UPMC system, a spokeswoman said. She said hospital officials were still gathering information on their conditions and identities, including the patients' ages.
The suspect was being questioned by county detectives and police at the Murrysville police station. Stevens said the suspect used a knife, though he didn't say what kind and said it wasn't immediately clear why the student attacked the others.
One student told WTAE he saw "students holding their stomachs, bleeding." That student wasn't sure how the assailant was stopped, but said at some point, a fire alarm was activated and said, "As soon as we heard the fire alarm was pulled we went outside."
Speaking outside the school, Morris Hundley said his 14-year-old daughter, Morriah, called him Wednesday morning in tears. Hundley came to the school still wearing his slippers, hoping for more information.
"My first thoughts were I think we need to home school now that this has happened," Hundley said. "The words can't describe how I feel. I'm just thinking of the victims."
Gov. Tom Corbett instructed state police to assist local investigators.
"I was shocked and saddened upon learning of the events that occurred this morning as students arrived at Franklin Regional High School. As a parent and grandparent, I can think of nothing more distressing than senseless violence against children. My heart and prayers go out to all the victims and their families," Corbett said in a statement.
School officials and Murrysville police didn't immediately return calls seeking further details, but the school issued a bulletin on its website saying: "A critical incident has occurred at the high school. All elementary schools are canceled, the middle school and high school students are secure."
The district later announced that the high school students were being moved to another school in the district, where their parents could pick them up.
The district serves about 3,600 students who live in the bedroom communities of Murrysville, Export and Delmont. The elementary and middle schools are part of the same campus.
Stevens said the incident happened after most students had arrived but before the school day was officially in session.
Initial reports indicated the stabbings happened during a fire drill, but investigators now believe another student may have pulled a fire alarm to alert others to the emergency in hopes that would clear the building.
Parents and former students say the school, which doesn't have metal detectors but does spot searches when security officials deem it necessary, generally does a good job of keeping students safe.
"I think the school reacted as fast ... as possible," said Rich Nickel, whose daughter, Jenna, 16, is a sophomore. "It's obviously a (lone) individual's act."