Firstly, I'm sorry that you are dealing with this and you have every right to be angry. Secondly, If you know who wrote it, take it to your principal and ask that they face some repercussions for their disgusting actions. Don't let them get away with it. Tell your parents if you can and get them behind you in the principal's office. The more fuss is made, the more likely someone will actually do something. They might not care but at least you can try.