The Secretary of Transportation may defer the service commitment of an individual under section 51306(a)(5) of this title (as specified in the cadet commitment agreement) for not more than 2 years if the individual is engaged in a graduate course of study approved by the Secretary. However, deferment of service as a commissioned officer under section 51306(a)(5) must be approved by the Secretary of the military department that has jurisdiction over the service or by the Secretary of Commerce for service with the National Oceanic and Atmospheric Administration.