There is a difference between court ordered child support and an order to pay for post majority expenses. Current child support by operation of law, 750 ILCS 5/505 terminates when the child turns 18. When the court order which set support was entered a date was required to be listed as to when support was to stop.
This date can be extended as a post majority expense and likely will be extended if the child now adult is still attending high school at the age of 18, and will continue until the adult turns 19 or finishes high school whichever occurs first. It does not extend to college without a petition being filed requesting post majority expenses for college pursuant to state law 750 ILCS 5/513.
Unlike child support the court has discretion to award these payments or not.