All contracts, hereafter awarded and entered into by any school district, shall contain a clause or stipulation requiring that no person shall be employed to do work under such contract except competent and first-class workmen and mechanics. No workmen shall be regarded as competent and first-class, within the meaning of this act, except those who are duly skilled in their respective branches of labor, and who shall be paid not less than such rates of wages and for such hours' work as shall be the established and current rates of wages paid for such hours by employers of organized labor in doing of similar work in the district where work is being done.