Westmoreland County emergency management spokesman Dan Stevens said no fatalities were reported.
At least four people with injuries Stevens described as "serious" were flown to area hospitals for treatment. He said some of the injured suffered stab wounds to the extremities.
Others were not actually stabbed, he said, and some of their injuries were cuts and scrapes.
The breakdown of injured and whether they were students or staff was not immediately known, but two hospitals reported nine patients ranging in age from 15 to 60.
Stevens said the alleged stabber, whose identity has not been released, was in custody.
Alyssa Finch, who said she is a student at Franklin Regional High School, told ABC News a student pulled the fire alarm during the stabbing "because he knew what was happening ... and he wanted the people to get out."
A statement posted on the Franklin Regional School District's website said "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."