Union School Principal Kurt Schweitzer was leading the eighth-grade graduation rehearsal in the Washington School gym in Rutherford around 9:30 a.m. when the student allegedly "ran across the gym and proceeded to thrust the knife, striking the chest of the principal several times," a Rutherford police news release said.
The 14-year-old then turned around and put his hands up in the air. A teacher took the knife and walked him outside to police, and he was taken into custody. His name was not released because he is a minor.
The boy and Schweitzer were treated for non-life-threatening injuries.
The teen is charged with multiple counts of aggravated assault and weapons possession. He remained in police custody as of Friday night.
No one else was harmed during the incident. It was unclear what might have motivated the attack.