Sec. 6. The several independent school districts and common school districts in such county shall continue to have authority to levy, assess, and collect the maintenance taxes theretofore authorized by the property taxpayers in said respective districts, subject to the restrictions that after said county-wide maintenance tax election has been carried and while said tax is in full force and operation, said respective independent school districts and common school districts shall not thereafter levy, assess, and collect any special tax for maintenance of schools, except in instances wherein the apportionment made by the Commissioners Court, together with the apportionment made by the State of Texas, produces an amount inadequate to meet the approved budget of such district, and in that event, such tax shall be levied in an amount to meet such deficit, due allowance to be made for delinquencies and for costs of collection. This law shall not affect the right and duty of said respective school districts to levy, assess, and collect taxes within their respective districts for the payment of principal and interest on bonded indebtedness of such districts. The respective districts shall continue to levy, assess, and collect taxes sufficient to pay principal of and interest on their bonds. Provided however, that nothing in this Act shall prevent the proper authorities from collecting and enforcing, for the benefit of the respective districts, any maintenance taxes levied before this law becomes effective.