How do I tell my teacher that I cut myself?

personally, I just showed him. My cuts were on my wrist so it wasn't that hard. Once we were alone I showed my teacher my wrist and he immediately understood. If you cut somewhere you can't show them, then it might help to take a picture, or to type/write it out, and if you're brave enough, just tell them honestly that you've cut yourself. in my personal experience, your teacher will ask you why and what led to this, but they'll know not to push it or overreact. They'll probably then refer you to someone who is trained to help you, so you can mentally prepare yourself to talk to a counsellor whom you may or may not be familiar with