I'm worried about my best friend

What worries me most is that looks like her arm.. Someone commuted suicide by hanging them self on at 2am on Tuesday morning at my school. I've seen her scars before, just last week she had lines all over her hand, they didn't look too good. We have a little friendship group consisting of 4 people and everyone except me jokes about it. If she ever brings stuff like that up over text I always remind her that she can talk to me and I'm there for her, but she never talks to me. I have social anxiety and I don't know if I could tell anyone, if I asked her first she'd most definitely say no. She's moving in a few weeks and might be leaving the school. I feel rather guilty because she's all hopeful with the person she likes and I like him too and.. It just a mess. I feel bad because I know she has some problems like this and I don't want to hurt her. She laughs and stuff at school, but I feel like sometimes it's just a mask. What do I do? Thank you for any advice, I really need it! By the way, we're middle schoolers, kind of young, but I'm still worried and she shouldn't be doing that no matter the age. I honestly don't know what to do, I'm worried she has problems deeper than I think and she needs someone to talk to but she won't open up. Help?