It is a defense to prosecution under Section 7–1‑17that a person appearing in a state of nudity did so in a modeling class operated:
(A) By a proprietary school, licensed by the State of Illinois; a college, junior college, or university supported entirely or partly by taxation;
(B) By a private college or university which maintains and operates educational programs in which credits are transferable to a college, junior college, or university supported entirely or partly by taxation; or
(C) In a structure:
Which has no sign visible from the exterior of the structure and no other advertising that indicates a nude person is available for viewing; and
Where, in order to participate in a class a student must enroll at least three (3) days in advance of the class; and
Where no more than one (1) nude model is on the premises at any one time.