What's the worst that would happen if they did know? Keeping it secret can be worse than being open (when appropriate). If you aren't in therapy, maybe they will suggest that you go for treatment. If you are coping with uni and handling your classes, there is no reason for you to withdraw. It is appropriate for classes about mental health to discuss self injury, because people need to understand it, and see it for what it is, and know how to deal with it appropriately - without judgment and with support. You don't have to tell them that you do it, although this could be an opportunity for you and your classmates to learn about support and caring and acceptance.

I was where you are now. One of my undergrad classes did talk about SI, and I was at the point where I was ready to stop hiding, after being so scared for so long. I emailed the professor and told her about it, and was honest with my classmates and answered their questions. I wasn't the only one in the class who self-injured. I wasn't done with therapy though, and now it is 6 years later and I'm still going to that professor for therapy. She understands SI and also overcame her own problems with it. Now I am also a therapist. I don't hide my scars anymore. I don't know if people guess what they are from - some are more accidental than others, and I can honestly say that those are from "fighting with my guanaco." It's the truth, for a couple of them. Not for all of them, and I should have stayed out of that guanaco pen, but oh well. My T doesn't go to any trouble to hide her scars either. Both of us wear short sleeves when it is appropriate (not too cold). People can think what they want.

You might not be ready to be that open about it, and that is fine. All I am suggesting is that maybe the fear of being found out could be worse than it actually happening.