Except as otherwise provided in this constitution, the total amount of general ad valorem taxes imposed upon real and tangible personal property for all purposes in any one year shall not exceed 15 mills on each dollar of the assessed valuation of property as finally equalized. Under procedures provided by law, which shall guarantee the right of initiative, separate tax limitations for any county and for the townships and for school districts therein, the aggregate of which shall not exceed 18 mills on each dollar of such valuation, may be adopted and thereafter altered by the vote of a majority of the qualified electors of such county voting thereon, in lieu of the limitation hereinbefore established. These limitations may be increased to an aggregate of not to exceed 50 mills on each dollar of valuation, for a period of not to exceed 20 years at any one time, if approved by a majority of the electors, qualified under Section 6 of Article II of this constitution, voting on the question.

     The foregoing limitations shall not apply to taxes imposed for the payment of principal and interest on bonds approved by the electors or other evidences of indebtedness approved by the electors or for the payment of assessments or contract obligations in anticipation of which bonds are issued approved by the electors, which taxes may be imposed without limitation as to rate or amount; or, subject to the provisions of Section 25 through 34 of this article, to taxes imposed for any other purpose by any city, village, charter county, charter township, charter authority or other authority, the tax limitations of which are provided by charter or by general law.

     In any school district which extends into two or more counties, property taxes at the highest rate available in the county which contains the greatest part of the area of the district may be imposed and collected for school purposes throughout the district.