Union School Principal Kurt Schweitzer was treated for injuries that weren't life-threatening, Lt. Marc A. Amatucci said.
The principal was leading his school's 8th-grade graduation in the Washington School gymnasium around 9:30 a.m. when the boy "ran across the gym and proceeded to thrust the knife, striking the chest of the principal several times," Amatucci said.
The boy then "turned around, dropped the knife and placed his hands in the air," the lieutenant said. "A teacher secured the knife and walked the attacker outside."
Officers Michael Merli and Thomas Steinel took the boy into custody and seized the pocket knife, he said.
Rutherford Ambulance Corps responded and treated both the boy and Schweitzer, who became the school's principal five years ago.
The boy remained detained Friday afternoon on a delinquency complaint charging him with two counts of aggravated assault and weapons offenses. The complaint will be heard behind closed doors in the Family Division of Superior Court in Hackensack.
A judge could require the boy to spend time in a juvenile detention facility and place him in a special program for juvenile offenders.
Detective Sgt. Michael Garner was investigating.
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