The following persons are exempt from the Barber Act while in the proper discharge of their professional or occupational duties: (1) Persons authorized by the laws of this state to practice medicine and surgery; (2) commissioned medical or surgical officers of the United States military services; (3) registered or licensed practical nurses; and (4) persons engaged in operating or employed in cosmetology establishments, except that nothing contained in this section shall authorize a cosmetologist to perform barbering as defined in section 71-202 in any licensed barber shop.