You need to be very careful when working with human tissue and pathogens. You could accidentally expose yourself or others to disease-causing bacteria and viruses. Biological waste, especially from humans needs to be disposed of in a safe manner and according to state guidelines. This means sterilizing all the petri dishes to completely kill all possible pathogens. In California, lab safety guidelines for schools are available on the web at: here

I recommend you to read from this handbook the pages that relate to your case. If you're not in California, your teacher should look up the guidelines for your state.