Teen-Age Exchange Students: Three young people shared a windy chat yesterday outside the new Ann Arbor High School. All are, or will be, taking part in the locally administered teen-age exchange program, "Youth for Understanding." Eckehart Bandemehr, (left) and Renate Rexhausen (right) are German students spending the year in Union City, Mich., and Ann Arbor, respectively. Patricia Gerow (center) is an Ann Arbor High School senior who will visit Germany this summer on the reciprocal half of the program. Eckehart is one of 25 foreign teen-agers spending this weekend in Ann Arbor.