The Juvies by Harlan Ellison
From the back cover:
Kick When He's Down! Aim For The Groin! Stomp Or Be Stomped!
These are the rules for survival in a kid gang. These are the rules that I lived by when I ran the gang myself, trying to find out what makes a JD tick. And let me tell you these kids aren't playing games. These are junior-grade killers.
I've pulled no punches in THE JUVIES either. I've painted these kids as they really are, in sharp black and white (and very often in blood red). Stark and stinking, and intended to scare the hell out of you.
New York: Ace, 1961. 189 pp. D-513.
Paperback. Very good.