The collector of taxes may and employes of the office of collector of taxes shall become members of the retirement system, the same as is provided in the case of other city officers and employes. The contributions required to build up the reserves necessary for the payment of city annuities to such officers and employes, shall be made, jointly, one-half each by the city and the school district within the territorial limits of such city. The board of school directors of each such school district are hereby authorized and directed to appropriate moneys for the purpose of making such contributions.