The obligation of the district to bargain in good faith with a duly designated or certified labor organization and to execute a written collective bargaining agreement with such labor organization covering the wages, hours, and working conditions of the employees represented by such labor organization in an appropriate unit, and to comply with the terms thereof, shall not be limited or restricted by any provision of law. The obligation of the district to bargain collectively shall extend to all subjects of collective bargaining which are or may be proper subjects of collective bargaining with a private employer, including retroactive provisions.