You wouldn't be a friend if you didn't help.
Self mutilation is a cry for attention. She needs you badly, even if she denies it. Everybody needs somebody. Maybe this is a way of making yourself feel better too.
Trust me, I know, that school and tests and homework and moving, all at one time, sucks and is putting you on emotional overload. It's a huge burden to bear. But you have to try to handle one thing at a time, one day at a time. I'm in the process of doing that now. You'd think at my age it would get easier or better, but it doesn't. But you can do it and you will do it and you'll be just fine. And that huge, ugly black cloud will disappear. {hugs}