If it is truly the parent’s fault and not the kids, then it’s the parent that should be penalized. I say fine ‘em after the second offense. Even a minimal fine of $30 bucks or so will get their attention. In the long run, even the most irresponsible parent will figure it’s cheaper to get a rebuilt battery for their car or whatever than to keep shelling out late fines.
They used to put that cardboard on the desks of the test cheaters, looking at other kids test papers for answers.