105 ILCS 5/6-19
Any vacancy on the regional board of school trustees shall be filled from the same territory by the remaining members until the next regular election for members of the regional board of school trustees, when the vacancy shall be filled for the unexpired time. A vacancy shall be subject to the residency provisions in Section 6-2 of this Code unless the vacancy occurs in a single county educational service region. If a vacancy occurs in a single county educational service region, then the vacancy may be filled by a person who is a resident of a congressional township not represented on the board. Removal of a member from the township from which such member was elected into a township which has its quota of members on the board shall constitute a vacancy.
105 ILCS 5/6-19