Nate Moore was walking to homeroom, book in hand, when a classmate he knew to be quiet and unassuming tackled a freshman boy a few feet in front of him. Moore thought it was the start of a fistfight and went to break it up.

But 16-year-old Alex Hribal wasn't throwing punches — he was stabbing his victim in the belly, Moore said. The suspect got up and slashed Moore's face, then took off down the hall, where authorities said he stabbed and slashed other students in an attack that injured 21 students and a security guard — and might have been even worse but for the "heroes" who Pennsylvania's governor said helped prevent further injury or loss of life.