The President shall establish a Foreign Service Schedule which shall consist of 9 salary classes and which shall apply to members of the Service who are citizens of the United States and for whom salary rates are not otherwise provided for by this subchapter. The maximum salary rate for the highest class established under this section, which shall be designated class 1, may not exceed the maximum rate of basic pay prescribed for GS-15 of the General Schedule under section 5332 of title 5. Salary rates established under this section shall be adjusted in accordance with section 5303 of title 5.