If someone went into a class knowing that it is being taught by someone closely related to economics, female empowerment and so forth, who would actually take some conveniently, or oddly placed bills? If there was something weird like a bunch of dollar bills laying on a table in such a class, you'd naturally assume it was a part of the class, and just completely avoid it. That's logic, reason, deduction.
The fact that they discussed it means that they are probably friends/know each other very well. Choosing to not sit near it indicates immaturity, or a lack of self control -- more likely, it was an exaggerated act by the girls to make it clear that they didn't want anything to do with it.
Their reluctance to accept a 20$ bill sounded kind of odd. It also sounded like something a group of tight knit friends that didn't want to be rude by taking the money for themselves would do -- so, instead of taking it for no other reason but greed, they came up with a goal met.
Neither scenario proves anything tangible.
If it was 20 million dollars, and it was extremely clear that the money was real, and someone would get it (IE; Not a joke.), it would be a different situation. Obviously.