The Federal Arbitration Act (FAA), based on rulings from the United States Supreme Court, trumps a variety of other types of laws. There is currently a split of authority between federal appeals courts as to the interaction between National Labor Relations Act (NLRA) and the FAA.
Some circuits, including the Ninth (which covers California) have found arbitration agreements that prevent class actions interfere with an employee’s NLRA rights to participate with their fellow employees to complain about wages and the conditions of employment. Other circuits have made the opposite determination.
It is expected the US Supreme Court will decide the issue soon.