46 U.S.C. § 51510

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).