Except as provided in section 254n of this title, each individual who has entered into a written contract with the Secretary under section 254l or 254l–1 of this title shall provide service in the full-time clinical practice of such individual’s profession as a member of the Corps for the period of obligated service provided in such contract. The Secretary may treat teaching as clinical practice for up to 20 percent of such period of obligated service. Notwithstanding the preceding sentence, with respect to a member of the Corps participating in the teaching health centers graduate medical education program under section 256h of this title, for the purpose of calculating time spent in full-time clinical practice under this section, up to 50 percent of time spent teaching by such member may be counted toward his or her service obligation.