No school bus used for the purpose of transporting school children shall be driven upon the ways of the state unless it carries the designation "School Bus" in a conspicuous place showing to the front and rear thereof in lettering not less than 8 inches in height, and has such other distinguishing marks as the director may prescribe. When a school bus is being driven upon a way for purposes other than the transportation of school children, all designating marks thereon indicating school bus shall be covered or concealed.