Dear friend,
Thanks for your letter. I am sad to hear that everyone in your class is against you. I remember you as a smiling boy with a great relationship with your classmates. I can't even imagine what have happened!
   First all, I want to advise you negotiate with your "friends" and clarify why do they hate you. Of course it will be hard, but if you want to regain your authority. Then, if it won't help you should ask your teacher or even principal to help you. Also you should rethink about a behavior, because the problem might be in you. 
  I am pretty sure that if you will complete my advises, you will not be more an object of fun. If you have more questions to me, just feel free to ask. 
  Hope everything will be alright.
  Best wishes,
  Slava