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Schools are required to allow students to be released for religious programs. This does not mean schools do not have the ability to create policies that protect the school's instruction time.
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(105 ILCS 5/26-1) (from Ch. 122, par. 26-1) Sec. 26-1. Compulsory school age; exemptions. Whoever has custody or control of any child (i) between the ages of 7 and 17 years (unless the child has already graduated from high school) for school years before the 2014-2015 school year or (ii) between the ages of 6 (on or before September 1) and 17 years (unless the child has already graduated from high school) beginning with the 2014-2015 school year shall cause such child to attend some public school in the district wherein the child resides the entire time it is in session during the regular school term, except as provided in Section 10-19.1, and during a required summer school program established under Section 10-22.33B; provided, that the following children shall not be required to attend the public schools:

5. Any child absent from a public school on a particular day or days or at a particular time of day for the reason that he is unable to attend classes or to participate in any examination, study, or work requirements on a particular day or days or at a particular time of day because of religious reasons, including the observance of a religious holiday or participation in religious instruction, or because the tenets of his religion forbid secular activity on a particular day or days or at a particular time of day. A school board may require the parent or guardian of a child who is to be excused from attending school because of religious reasons to give notice, not exceeding 5 days, of the child's absence to the school principal or other school personnel. Any child excused from attending school under this paragraph 5 shall not be required to submit a written excuse for such absence after returning to school. A district superintendent shall develop and distribute to schools appropriate procedures regarding a student's absence for religious reasons, how schools are notified of a student's impending absence for religious reasons, and the requirements of Section 26-2b of this Code;

All school districts, regardless of whether they are required by law to allow released time, should implement good policy and be prepared for when a released time program may approach the district. The time to set the policy is before an active program is pressuring the district. Research polices the district can implement to protect teachers' and students' instruction time. Set polices that limit disruptions to students and the ability for proselytization to occur, impacting other students of faith or no faith. A more detailed write-up on school board policies can be read here.