Why doesn't my boss believe me-- what do I do?

A few weeks ago, my boss confronted me about another teacher complaining to me that I had led his students to rebel against his grading methods during second period. I explained to my boss that, not only do I not have a second period, but I wasn't even at the school. She seemed to have believed me when I explained this and dismissed me. I asked which students were saying this but she refused to answer and told me not to worry about it. Today, I stopped by work to have my boss sign a completely unrelated document, and she brought the issue up once again. She doesn't believe me, and I feel this will effect my schedule, and if there is another issue that comes up, I will most likely lose my job. I was not working at that time. She doesn't believe me. What should I do?