The Secretary of Transportation may defer the service commitment of an individual under section 51509(d)(5) of this title (as specified in the agreement under section 51509) 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 on active duty must be approved by the Secretary of the affected military department (or the Secretary of Commerce, for service with the National Oceanic and Atmospheric Administration).