Added: I'm going to respectfully disagree that we should turn our heads when cheating happens and accept it.
The trick is to find the best strategy to stop it.
I, for one, was not suggesting any such thing. I was pointing out that there's nothing that can reasonably be done in this case, at this stage.
Since it seems to be necessary to spell it out: Stephanie has only her DD's word that the cheating
wasn't the other way round. The only piece of evidence in the case is the identity of the answers, and that is symmetric. I'm sure she knows her DD well enough to be sure, but it would be another matter to expect the school to act on that basis!
I'm actually personally most inclined to be angry with the school for putting students in a position where they were highly likely to see one another's answers accidentally, and thereby be exposed to a strong and unnecessary temptation. I imagine the organisers of the competition would also be unimpressed that children sitting it were sat at the same table as one another. [ETA and if they were informed of this case, the only thing they could reasonably do would be to disqualify both girls. So I don't suggest that!]