A crazy girl at school is trying to ruin my life?

today i was sat at lunch with my friends when one of my friends came up to me and said that a girl is talking sh** about me over on another table. he told me she was saying that she caught me writing into a table that i hate her. i didnt know what to think so i went over and asked her if she thought i did that. she would only say go away or leave me alone whenever i said anything but i told her that she is obviously lying to get attention and its clear she is lying because she wouldn't repeat the story to me. i thought that was the end of it. then i find out she told the headteacher something. i dont know what. but he has been going around asking people if they saw what happened between me and her this lunchtime. i dont know what she told him. i dont know what her problem is with me. what am i supposed to do?