So, if you actually bothered to look into the laws of Illinois, you'd find that a school zone sign says the following:
"SPEED LIMIT 20 ON SCHOOL DAYS WHEN CHILDREN ARE PRESENT"
No flashing lights indicating when (e.g. in Ohio, it's a school zone when the MPH is lit and the yellow lights are flashing), and so on. To add to the confusion, good luck finding "regular" (non-school zone) signs in Chicago. Supposedly that's 30 MPH when no sign is present, but unlike the suburbs, they don't have that info on signs at the city border...
To add, back in the early 80s, due to the confusion over this sign, a state attorney general put out some guidance saying that a police officer needs to see a "student" (e.g. a child under 18) within eyeshot, when school is in session. These speed cameras don't do that. (Of course, state law could be changed to have times or flashing lights, but that hasn't happened). Then, Rahm & the camera companies wanted to put wide-angle lenses to see what "children" could be found, but then parents started filing lawsuits about faceless red-light camera companies taking pictures of their children, for the benefit of the camera companies. (Not sure what happened with that...)
To add, about a year ago, a lawyer filed a class action on these cameras, because people got school zone speed camera tickets on a Sunday evening in July...
So, sooner or later, a court will rule against the city, and quite badly... Heck, refunds are already happening en masse...