105 ILCS 5/19-21
If there is no default in payment of the principal of or interest upon the refunding bonds, and a sum of money equal to the amount of interest that will accrue on the refunding bonds and a sum of money equal to the amount of principal that will become due thereon within the next 6 months period has been set aside, the treasurer of the school district shall use the money available from the proceeds of taxes levied for the payment of the refunding bonds in calling them for payment, if, by their terms, they are subject to redemption. However, a school district may provide in the bond resolution that whenever the school district is not in default in payment of the principal of or interest upon the refunding bonds and has set aside the sums of money provided in this section for interest accruing and principal maturing within the next 6 months period, the money available from the proceeds of taxes levied for the payment of refunding bonds shall be used, first, in the purchase of the refunding bonds at the lowest price obtainable, but not to exceed their par value and accrued interest, after sealed tenders for their purchase have been advertised for as may be directed by the corporate authorities thereof.
Refunding bonds called for payment and paid or purchased under this section shall be marked paid and cancelled.
105 ILCS 5/19-21