No contract or agreement shall be made with any labor organization, association, group, or individual, or be assumed under the provisions of this section, where such organization, association, group, or individual denies membership to or in any manner discriminates against any employee on the grounds of race, creed, color or sex; provided, that such organization may preclude from membership any individual who advocates the overthrow of the government by force or violence. The district shall not discriminate in regard to employment against any person because of his race, creed, or color.