Scribble on a Page
This one might help as a short term distraction, and something you could use while you’re in school. Scribble on a page until it’s completely covered. It might help you to vent some feelings of anger. And by the time the page is totally blacked out you might find the urge to self-harm has passed.
It’s important to find what works for you. If this doesn’t help, don’t worry. There are plenty of other ways to cope. See other ideas of things to try.