105 ILCS 5/10-20.12
To establish and keep in operation in each year during a school term of at least the minimum length required by Section 10-19, a sufficient number of free schools for the accommodation of all persons in the district who are 5 years of age or older but under 21 years of age, and to secure for all such persons the right and opportunity to an equal education in such schools; provided that (i) children who will attain the age of 5 years on or before September 1 of the year of the 1990-1991 school term and each school term thereafter may attend school upon the commencement of such term and (ii) based upon an assessment of the child's readiness, children who have attended a non-public preschool and continued their education at that school through kindergarten, were taught in kindergarten by an appropriately certified teacher, and will attain the age of 6 years on or before December 31 of the year of the 2009-2010 school term and each school term thereafter may attend first grade upon commencement of such term. However, Section 33 of the Educational Opportunity for Military Children Act shall apply to children of active duty military personnel. Based upon an assessment of a child's readiness to attend school, a school district may permit a child to attend school prior to the dates contained in this Section. In any school district operating on a full year school basis children who will attain age 5 within 30 days after the commencement of a term may attend school upon the commencement of such term and, based upon an assessment of the child's readiness, children who have attended a non-public preschool and continued their education at that school through kindergarten, were taught in kindergarten by an appropriately certified teacher, and will attain age 6 within 4 months after the commencement of a term may attend first grade upon the commencement of such term. The school district may, by resolution of its board, allow for a full year school plan.
105 ILCS 5/10-20.12