No person shall practice as a dental hygienist in Missouri as defined in section 332.091 except under his own name and unless and until the board has issued to him a certificate certifying that he has been duly registered as a dental hygienist in Missouri and unless and until the board has issued to him a license, to be renewed each period as hereinafter provided, to practice as a dental hygienist in Missouri; but nothing in this section or chapter shall be so construed as to make it unlawful for a duly registered and currently licensed dentist in Missouri to practice as a dental hygienist in Missouri, or to make it unlawful for a qualified dental hygienist legally licensed in a state other than Missouri to give a demonstration or demonstrations before a meeting of dentists or dental hygienists or both in Missouri, or to make it unlawful for a legally qualified and licensed physician or surgeon to practice such oral hygiene as is a recognized part of the practice of medicine or surgery, or to make it unlawful for dental hygiene students in any accredited dental hygiene school to practice as a dental hygienist under the personal direction of instructors, or to make it unlawful for persons to practice as dental hygienists in the United States armed services or in the United States Public Health Service, or in or for the United States Veterans Bureau, or to teach in an accredited dental hygiene program, or to make it unlawful for dental assistants, certified dental assistants or expanded-functions dental assistants to perform polishing of teeth.