It's the sort of thing you don't hear much about unless you've worked a job in teaching or childcare. Basically it's teenagers, usually girls, who cut themselves to feel alive after some sort of trauma, or because they see their other friends do it and join it. The progenitor in most cutting groups was most likely molested or came from another group of cutters when they moved schools. The standard places for cutting are between the fingers and toes so it's easier to hide from teachers and parents. Sometimes high up on the thighs for the same reason. I encountered a cutting group at a job about a decade ago. The normally method for dealing with it is to hand the cutters off to a more experienced/trained adult and break the friend group up. Isolate the first cutter and get her some help. This is not the first time I've seen it almost being encouraged in media, but this is the most mainstream example I can think of. If you ever encounter a teenager who casually mentions "self-harm" to you, get away from them and let someone in authority over said teenager know immediately to protect yourself. You do not want to tangle with that Tar Baby by yourself.