You stated that you can’t tell your parents but I would ask you to reconsider. Many teenagers feel that they can’t speak to their parents when in reality they simply haven’t tried.
I would strongly urge you to tell your parents how you’re feeling. It is a major concern that you are cutting, experiencing severe depression, experiencing physical pain, and possibly failing your coursework. Those are all matters that should be brought to the attention of your parents. It is simply a matter of time before your parents find out about these issues. Thus it’s better that they know sooner rather than later.
If telling your parents is not an option, then try the school counselor. I understand that you are frightened to speak to the school counselor but you shouldn’t be. It is their job is to assist with students’ problems.
One final option is to write a letter to your parents or to the school counselor. Some people feel more comfortable expressing themselves in writing rather than face-to-face contact.
Please understand that it is vitally important that you ask for help. You cannot be helped if those around you are not aware of your suffering. Speak to your parents, the school counselor or with someone you trust. Many teenagers feel the same way you do but later realize that asking for help was much easier than they had expected. Please take care.