Other than a cottonmouth and a rat snake both typically being black in color (not always), I don't remember a whole lot in common between the two that would trigger a misidentification, especially if they were sitting there in a classroom being closely inspected (in an aquarium, small cage, or something small like that). The triangular head, the pits in front of the nostrils, the short fat body, the white inside of the mouth - sheesh, what a science teacher!