Ok, so I haven't yet read all of the comments so I apologize if this is redundant. So, I teach Middle School and let me tell you 7th and 8th grade are literally the toughest ages. This is the age of the most changes, the most insecurity, the most hormones ect. Kids today are under a social pressure that we were never under. Due to the internet and social media teens are under a near constant flow of info and social exchanges that they almost never get away from. Teens at this age especially possess insecurity and need validation especially, but they simply do not get it from their peers and unfortunately often from teachers.
Shows like 13 Reasons Why, while great, because it sheds light on important issues, can create ideas like self harm and drinking at school to be glamorous and trendy. I have an advisory and I hear from my girls about the trendiness of self harm especially at age 13. There is immense pressure and anxiety right now among girls, women and minorities and we see this in school. There a MAJOR increases in bullying and it PEAKS in Middle School.
So, while self harm is serious and cause for concern, it also is normal and even trendy. I often tell parents that ms kids are in such a state of change that they will try various behaviors on like trying on clothes. These behaviors do not define who they are. Middle school students on the whole struggle with emotional regulation and cutting is a behavior that is talked about and attempted frequently.
Her "tribe" (peer group) has MAJOR influence over her behaviors. It is good she is joining band for a new social group and band will help her identity and self esteem.
I like the book "Untangled: Guiding Teenage Girls Through the 7 Transitions." She talks about the tribe and what to do when your daughter deals with tough things (like drugs, sex, alcohol, social pressure).
She does talk about cutting and does suggest therapy.
I get the guilt and stress over this. I have had to be in intense therapy for years but this makes me a better mom.
I would suggest a teen group or a teen DBT group with sliding scale or getting a teen DBT work book you could do together. I would also suggest telling her your story. She is old enough to not only understand, but would be benefitted by it. You would be showing her you are not perfect and that you are in therapy working on yourself. She would know that if you seem distant it is about you, not about her. You can be an anxious mom, but you are managing it. Her behavior is not your fault. However, the key is validation to easing self harm and that is what you have already done.