Illinois Child Support, Employer withholding too long?

My youngest that I pay CS on just graduated High School, last day was May
13th. I asked my employer if the order for withholding had an end date on
it, they looked it up and found an end date of 12/9/2010, the Childs 18th
birthday. From my understanding child support was supposed to last until
they're out of high school. Anyway, my employer also withheld CS last week
too, I don't like their "no big hurry" attitude, they'll stop withholding
when they get around to it.

Should child support have ended on the court ordered date of the Childs 18th
birthday or when she got through with high school?

I'm thinking if my employer could be held responsible for the money they
withheld since ordered date of 12/9/2010, they might be a little more
motivated and responsive to discontinue the withholding. Of course them
owing $5k doesn't sound too bad either.

RogerN