105 ILCS 5/34-44.1
When the value of the taxable property in the school district is increased either by the establishment of a new multiplier by the Department of Revenue or the availability of a later assessment by virtue of additions to the tax rolls or increases in assessments made by the county assessor after the annual school budget has been adopted, the board of education may adopt, by a 2/3 vote of the full membership of the board, a supplemental budget to provide for the use of the added potential revenues in an amount that shall not exceed a sum equivalent to the product of the amount of the increase in the value of taxable property in the district multiplied by the maximum per cent or rate of tax which the board and the corporate authorities of the city are authorized by law to levy for the current fiscal year for educational, building, free textbook, agricultural science school, supervised playground outside school hours purposes (or supervised playground outside school hours and stadia, social center and summer swimming pool open to the public purposes, as the case may be) or special education purposes. With respect to any supplemental budget based upon an increase in the value of the taxable property in the school district, such supplemental budget shall be adopted within 60 days of the date of the final certification of the equalization rate by the Department of Revenue to the county clerk as provided in the Property Tax Code, regardless of whether the adoption occurs within or after the close of the fiscal year to which the increase applies, but shall not become effective unless approved in accordance with Article 34A of "The School Code".
105 ILCS 5/34-44.1