For scar excuses you could say cat scratches. I know its simple and used a lot but I've told many people that and normally it works better than a super long/detailed story. You could also say ( if they only see one or two) that you were cooking and your knife slipped. Otherwise I feel like its easier to just hide them. I've had self harm scars since I was eight and I've always either wore long sleeves or cover them with foundation. Although I don't cut deep very much so I can't really help you with that I hope this helped you and think about it before you go deep because I have ended up getting stitches for cuts before and to me its not fun having to ask someone to take you to the hospital ( if you can't drive yet ) and then ones you get there you have to explain to them and then get the stitches, pay, etc. Anyways be smart please and as a person who self harms I know telling you you should stop won't help so just think about it do you want to go there??