236.31  District millage elections.--The school board, pursuant to resolution adopted at a regular meeting, shall direct the county commissioners to call an election at which the electors within the school districts may approve an ad valorem tax millage as authorized in s. 9, Art. VII of the State Constitution. Such election may be held at any time, except that not more than one such election shall be held during any 12-month period. Any millage so authorized shall be levied for a period not in excess of 2 years or until changed by another millage election, whichever is the earlier. In the event any such election is invalidated by a court of competent jurisdiction, such invalidated election shall be considered not to have been held.