Maybe you could use some cover-up/foundation stuff? I do that sometimes when I want to keep some of my bad scars covered, and it usually goes unnoticed, depending how bad the scar is. Maybe make a story up about how it happened, and stick to it. OR - you could just say it's a scar from a long time ago...can't even remember how it happened, or something like that.
People have jokingly said things like "oh, are you a cutter now?" and crap like that, and yeah, it makes me mad, but I know that if I chortle and say "oh yeah, totally" in a sarcastic tone they will usually leave it alone. You don't owe it to ANYONE to have to explain why it happened, so if you don't feel comfortable lying, make a joke of it and move on.
Ugh, I know these fears all too well...and it sucks
Wish I had more advice to give. Good luck!
-Alicia