Its calming because your body is releasing calming chemicals when you cut. Its a body defense mechanism, such as when you're injured, it doesn't help you to get hysterical, rather, it helps you to be calm so that you can get through it all. That is, we are designed to be calm when injured in order to survive.

Can you talk to an adult or counselor at school? I have read a number of your posts and it breaks my heart to hear about how you don't have support at home. [I am a recovering self-injurer myself.]