The overall goal of this study is to compare the effects, good and/or bad, of the chemotherapy drugs carboplatin and isotretinoin on subjects with high risk medulloblastoma and to find out if one or both are better than standard therapy alone. The term, risk, refers to the chance of the cancer coming back after treatment. Standard therapy for medulloblastoma may include surgery, radiation therapy to the brain and spinal cord, and chemotherapy. Radiation therapy is the use of high-energy x-rays to kill cancer cells. Chemotherapy means treatment with ¿cancer killing¿ drugs.