Loans shall be for terms that do not exceed thirty years and shall bear interest at (a) a rate determined by the Secretary of the Treasury taking into consideration the market yield on municipal bonds: Provided, That in no event shall the rate be greater than the rate determined by the Secretary of the Treasury taking into consideration the current average yield on outstanding marketable obligations of the United States of comparable maturity, plus (b) such additional charge, if any, toward covering other costs of the program as the Secretary may determine to be consistent with its purpose: Provided, That educational loans may provide for interest to be deferred while the borrower is in school or in the military service.