105 ILCS 5/17-12
When a district lies partly in two or more counties the school board shall ascertain, as near as practicable, the amount to be raised by special tax for educational and operations and maintenance purposes and shall prepare a certificate for each county in which the district lies and shall deliver one of such certificates to each of the county clerks of the counties in which a part of the district is situated. On the first Monday following the delivery of the certificate, or as soon thereafter as may be practicable, each county clerk shall ascertain the total equalized valuation of all the taxable property in that part of the district as lies in his county, and certify the amount thereof to the county clerk of each of the other counties in which any part of the district lies. From the aggregate of such equalized valuation and from the certificate of the amount so required to be levied, such clerk shall ascertain the rate per cent required to produce in the district the amount of such levy, and at that rate shall extend the special tax to be levied for educational and operations and maintenance purposes in that part of the district lying in his respective county.
105 ILCS 5/17-12