I would agree with
>>2 about finding the culprit first and based on as much evidence as you can get. Then, well, it depends on what your personal feeling is about what should be done to him. You could just forgive him or if you feel the need for a penalty try to contact a teacher you can trust or maybe your parents (I don't know your relation to them, so that could not work out). In my opinion it is best for you to decide what you feel is right and try handling the issue according to that. You may not like the result and thus have the chance to learn about yourself some more. It could even get dangerous for you (I don't know how agressive or brutal the people at your school are) so please bare in mind that you should be careful when charging someone.
Just don't get yourself into to much trouble because of this, not worth it. Hope my view can affect your situation positively, I just want to help.