I just wanted to say that the fact you know and she hasn't denied it is a huge step forward. It signifies that she trusts you and that she may want to find a different way to express herself - both of which are good things of course.
I work very closely with Rethink and they provide some really excellent resources so I second that suggestion.
I was a cutter, up to 30 times a day at my worst, and I had a weekly appointment with the nurse at my GP surgery to check and dress the wounds. It was never my intention to go too deep but it did happen by accident a few times, so encouraging her to show you without fear of reprisal is really important, and it sounds like you're already doing that.
From time to time I have seen young people cut because of a 'trend' but really the trend is just not knowing how to deal with feelings that are new and sometimes overwhelming. We all experiment during our teenage years - the very nature of it is that you feel judged all the time, and any difference is picked up on by others also struggling with their own differences.
If there's a boy behaving that way then you definitely need to inform the school, but also just make sure your daughter knows the lines of communication with you are always open, and reassure her that you'll be there to listen without judgement whenever she needs you.
Seek support yourself also - it's so important not to forget about your own mental health in all this.
Cutting is very very scary for those who care looking in, but as hard as it feels, keep in mind that many people harm themselves in less visible but far more damaging ways, and you're better able to help with something you can see. I realise that sounds ludicrous but it really is true.
I really wish you and your daughter the best :)