The clip shows one individual chasing her down as she falls at the corner of an intersection, before he punches and kicks her. A second male comes running in, jumping high into the air to stomp on her with both feet.
Within seconds 16 more individuals seemingly materialize out of nowhere, shoving at each other to try to get a kick in.
Just seconds later they disperse as rapidly as they congregated, appearing to laugh as they flee.
The final assailant can be seen pulling the Nike trainers off her feet as she lies helpless on the pavement.
The victim was taken to hospital, and is expected to recover from her injuries.
NYPD Crime Stoppers offered a reward of up to $2,500 for information on the attackers.
"For so many kids to jump on one individual, it's just ridiculous," witness Carlita Gorden told CBS New York.
"Her book bag, no shoes, she was just sitting on the floor with no shoes. Then you see the little one holding her sneakers, running away. Come on, it's sad. We have to do better as a community. We have to do better as people in general. That this little girl could've lost her life is really sad."
Registered nurse Anita Peavy told the station she helped the victim: "She was bleeding like crazy. We just tried to keep her calm and sitting there 'til the ambulance come."
"I have never witnessed something like that yet in my life... such a small kid dropped by another kid, was crazy."
Sharing the disturbing video on Twitter, NYPD Assistant Chief Jeffrey Maddrey wrote: "This is sickening video of a 15 year old girl viciously attacked by a group of school children. One young man takes the sneakers right off the unconscious victim's feet."
"The teenager is in the hospital recovering. We CAN NOT allow this behavior in our community. #Speakup"