Sec. 7-11-20 Exemptions.
It is a defense to prosecution under Section 7-11-17 that a person appearing in a state of nudity did so in a modeling class operated:
   (a)    By a propriety school licensed by the State of Wisconsin or a college, junior college or university supported entirely or in part by public taxation;
   (b)   By a private college or university which maintains and operates education programs in which credits are transferable to a college, junior college, or university supported entirely or partly by taxation;
   (c)   Or in a structure:
      (1)   That has no sign visible from the exterior of the structure and no other advertising that indicates a nude or semi-nude person is available for viewing; and
      (2)    Where in order to participate in class a student must enroll at least three (3) days in advance of the class; and
      (3)    Where no more than one nude or semi-nude model is on the premises at any one time.