Despite what everyone here says, self-injurers rarely do it for attention. Why would you physically hurt yourself when there were so many other ways? Sometimes though, it is a cry for help. You can read about that here. http://psychologytoday.com/articles/pto-20040521-000002.html
Since she's letting you see her cuts, chances are she wants you to say something. She's crying for help but wants someone to come to her rescue instead of having to ask. When people do that it's usually a test to see if people care about that much.
Say something. You don't even have to wait until next time. Find her at a time when she's alone (don't do this in the middle of class) and say, "Can I talk to you for a sec? I just noticed that you have marks on your arms and I was wondering about it. Are you ok?"
Even then she might become embarrassed that she let you see them at all and deny it or do so for other reasons. That might sound weird, but it happens. If she does say, "I've seen [cat scratches/whatever other excuse] and they don't look like that."
And then let her talk about it.
I know this might seem to be asking a lot for someone it sounds like you don't even know well, but she chose to show them to you meaning she probably wants to share this with you. It would be the right thing to do.
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