105 ILCS 5/3-14.18: Map - Numbering of districts.

To keep in his office a map of his county on a scale of not less than two inches to the mile and to indicate thereon the boundary lines and numbers of all school districts. Districts shall be numbered consecutively. If a new district composed of parts of two or more counties is formed, the county superintendents of such counties shall agree upon a number by which the district shall be designated, which number shall not be a duplicate of any number in either of such counties.