[~Misguided Ghosts~]

Perhaps you can excuse yourself to take a walk to the bathroom or to the water fountain? I am saying this not to tell you that you should harm yourself, but because sometimes taking a short walk can give you a moment to get away from whatever the situation is and take a breather, which may help. If it's not too disruptive to the functioning of the class you can also do something like doodle or write something out in a notebook. Sometimes I'd quietly rip up or put holes in a piece of paper in the notebook just to get things out.

I do urge you to talk to someone like a guidance counselor though when you get these urges. They are mandatory reporters, but they would know how to help and that way you wouldn't be dealing with this alone.