The legal voters of any Class VI school district may, by a fifty-five percent majority affirmative vote of those voting on the issue at a special election of the district, extend the grade offerings of that district to include grades seven and eight or grades six through eight. Such election shall be conducted by the county clerk or election commissioner in accordance with the Election Act. If the issue receives such fifty-five percent majority affirmative vote, the school district shall then be known as a Class VI junior-senior high school district and shall be supported in the same manner as was provided for the support of the district previous to the extension of its grade offerings. In such an election, the legal voters of all Class I school districts in which there is located an incorporated city or village shall vote separately and the remaining Class I school districts shall vote separately as a unit either for a plan for the individual district or with more districts as determined by the vote by the election. Fifty-five percent of the votes cast in each voting unit shall be in favor of the proposition to put such a plan into operation.