Sometimes, things can change in an instant. A slim 13-year-old Grade 7 can step out of his classroom and be struck by the bigger, bulkier, stronger Grade 8 sprinting bullishly down the hall. The Grade 8, shoulder at the level of the Grade 7's head, can make clean contact with the side of the younger boy's skull, forcing it to snap and spin back into the brick wall behind him before he crumples to the floor.
Head meets shoulder, wall, and floor, in the blink of an eye.
Maybe, even, the younger boy could be knocked out by one of the three blows, eyes rolling back into his head.