A woman has admitted striking and killing a high school senior in March of last year in Montgomery County. 20-year-old Stephanie Phillips of Philadelphia pleaded guilty to leaving the scene of a fatal accident.  District Attorney Risa Ferman said at the time that the death was clearly accidental, but Phillips was charged criminally because she knowingly hit someone and drove away.  She admitted hitting 17-year-old Denise Cotteta of Plymouth Meeting as the victim stepped onto Germantown Pike near Plymouth-Whitemarsh High School during a late-night rain storm, but she did not cause the accident and she was not under the influence of drugs or alcohol.  Police said the victim consumed a small amount of alcohol while celebrating the school’s state basketball championship with friends, but she was not drunk.