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: http://www.cde.ca.gov/ci/SciSafety.pdfIt recommends autoclaving the petri dishes 3 times over three days to kill all resistant fungi and bacteria. If you don't have an autoclave, you can use a pressure cooker, but be sure to have an adult help you. The school nurse might also have sterilization equipment. If you're not in California, your teacher should look up the guidelines for your state.