40 ILCS 5/15-113.2
"Service for leaves of absence" includes those periods of leaves of absence at less than 50% pay, except military leave and periods of disability leave in excess of 60 days, for which the employee pays the contributions required under Section 15-157 in accordance with rules prescribed by the board based upon the employee's basic compensation on the date the leave begins, or in the case of leave for service with a teacher organization, based upon the actual compensation received by the employee for such service after January 26, 1988, if the employee so elects within 30 days of that date or the date the leave for service with a teacher organization begins, whichever is later; provided that the employee (1) returns to employment covered by this system at the expiration of the leave, or within 30 days after the termination of a disability which occurs during the leave and continues this employment at a percentage of time equal to or greater than the percentage of time immediately preceding the leave of absence for at least 8 consecutive months or a period equal to the period of the leave, whichever is less, or (2) is precluded from meeting the foregoing conditions because of disability or death. If service credit is denied because the employee fails to meet these conditions, the contributions covering the leave of absence shall be refunded without interest. The return to employment condition does not apply if the leave of absence is for service with a teacher organization.
Service credit provided under this Section shall not exceed 3 years in any period of 10 years, unless the employee is on special leave granted by the employer for service with a teacher organization. Commencing with the fourth year in any period of 10 years, a participant on such special leave is also required to pay employer contributions equal to the normal cost as defined in Section 15-155, based upon the employee's basic compensation on the date the leave begins, or based upon the actual compensation received by the employee for service with a teacher organization if the employee has so elected.
40 ILCS 5/15-113.2