One of the students in the show, Juliette, cuts herself.
All the kids in the show have a history of some sort of drug use, child abuse, etc. The show focuses on the most troubled kids at Mount Horizon, a rehabilitation school in the Pacific Northwest where troubled teens who have been abused, involved in drugs, prostitution, or gangs learn to grow in a safe environment. The series focuses on a group of students called the Cliffhangers, each with their own story and troubles.
Quote from Juliette when caught self-injuring: It makes the pain inside me go away. So that I don't have to think about it. It lets it leak out, where everyone else can see it.