What do teenagers want in a doctor or dentist? A survey of more than 6,800 ninth-graders in 39 Philadelphia public schools, published in today's Journal of the American Medical Association, found that their top concern is clean hands, followed by clean instruments. Reason: the students are afraidthey may get AIDS."It surprised all the experts, because the experts did not know that teen-agers had this misconception," said the lead researcher on the survey, Dr. Kenneth Ginsburg of the Children's Hospital of Philadelphia. The facts: of the estimated 1.5 million HIV-infected Americans, only six are believed to have contracted it from a health care professional -- all from the same dentist.