By Yelena Mandenberg & Yelena Mandenberg A 15-year-old girl was arrested on unknown charges after authorities determined that she was ultimately responsible for a schoolyard fight that left one girl in the hospital with a head injury. A dread-inducing video went viral on social media, showing a 15-year-old girl being knocked down by a fellow teen in leggings and a black shirt. In a terrifying scene, more kids crowd around and start kicking at the already downed girl. Then they move out of shot, leaving the initial attacker to pin the victim to the ground, pounding her head on the concrete. The…