Section 18. The teachers’ retirement board shall appoint and may remove an executive director and shall establish his salary. Subject to the approval of said board, the executive director shall employ other employees, prescribe their duties and fix their compensation; provided, however, that the salaries of said employees shall not exceed the sum annually approved therefor by said board. The provisions of sections nine A, forty-five, forty-six and forty-six C of chapter thirty, chapter thirty-one, and chapter one hundred and fifty E shall not apply to any employee of the board.