My friend and I are both teenagers, and we've known each other for few years but never been really close. She's a girl, I am a boy, and we go to the same school. We hung out yesterday, and I saw a series of cuts on her right forearm. They looked like deliberate cuts because they were perpendicular to her arm and regularly spaced. I didn't mention it to her because I am still deciding what to do.
I spent some time online reading about cutting last night, and I know the cutting is because of emotional pain and it can be addictive. I don't cut or harm myself, nor do I intend to in the future, but I know what it's like to have unbearable pain and I want to help her, but I don't really want her to be punished or anything for her cutting. I'm not sure whom I can tell, but I know that I should help her. Should I tell her first? Just so you know, I'm afraid of doing that. Should I tell a counselor at my school, or something else?

You should tell her parent first.They are the one who can handle the situation more than anyone else. Tell them what ever you observed and then wait and observe the situation. In case if you will not see any improvement over the situation then tell your counselor about it. But right now, just call or meet her parents.
God bless u

Is she left-handed? You might want to ask her what happened first, it could have been an accident. Speaking from experience, her parents should be the last ones to speak to about it. Seeing someone on a personal level everyday that knows what she is doing may put more pressure on her. I would speak to a counselor first and let them decide what to do. In the meantime, just continue being a good friend.

Thanks for the advice. Telling her parents seems like the best idea, but I don't know if I have any means of contacting them. I'll see what I can do, but I might have to tell a school counselor instead.

Is she left-handed? To make the cuts on her right arm... If she is right handed, it is doubtful that is what is happening. Having a blade in the hand that she is not accustomed to using is one thing, but to cut herself on the arm that she uses most making it more visible is highly unlikely. My parents tried to 'bully me' into stopping strange behaviour and it got worse. Just don't lie to her if she asks about you telling. Stand up and admit it.

DO NOT TELL ANYONE AT ALL IF YOU Haven't TALKED TO HER ABOUT IT FIRST, AND ONLY IF SHE IS OKAY WITH IT, TRUST ME. If it were me itd make me want ot do something worse. Don't tell her you understand, because if you haven't done "it" you really cant, she probably is cutting herslf, sorry to say it but that's exactly what it looks like, it feels good to have people you like ask about it, so ask her straight up. Id have probably stopped a long time ago if someone seemed to care.