Twenty students have been stabbed in an incident at a Pennsylvania high school.

There were no fatalities reported after the attack at Franklin Regional High School in Pittsburg, but four students are considered to have significant injuries.

The four most seriously injured were airlifted to hospital, with the sixteen other injured students taken by ambulances to nearby hospitals.

The school was put on “lock-down” while police scoured the buildings for the attacker.

A male student has been arrested and is being questioned over the attack.

A statement on the school’s official website from assistant superintendent Mary Catherine Reljac reads:

After a critical incident at Franklin Regional Senior High School this morning, the campus was locked down. Elementary classes were cancelled and high school students were evacuated to the middle school. Law enforcement are working with the school district to manage the situation.

Middle school and high school students will only be dismissed to parents. Parents should report to Heritage Elementary for reunification. All parents will need to sign out students before dismissal. As parents arrive at Heritage Elementary, staff members will escort middle school and high school students to the building for reunification. Any high school student who drove to school will not be permitted to drive home without a parent.