40 ILCS 5/17-136

The aggregate sum required to be contributed by any person otherwise entitled to a pension under this Article shall be computed as follows: 1. For service as a teacher after July 6, 1931, by computing the total of the required deductions at the rates in force when the service was rendered and required contributions for optionally creditable service at rates prescribed in this Article. 2. For service as a teacher after September 1, 1963, by computing the total of the required deductions at the rates in force when salary was paid and required contributions for optionally creditable service at rates prescribed in this Article. Effective September 1, 1974, any deficiencies resulting under the provisions of this section shall accrue at an interest rate of 5% per annum compounded annually until paid.