That's a toughy!
According Hanlon & Messenger (Cephalopod behaviour) cephalopods are quite sensitive to low frequency vibrations (~10Hz) but there are arguements over whether or not this can be called "hearing". It's really a problem of definitions. Cephs have no air filled cavities with associated structures that can detect the pressure waves of sound! BUT the statocyst chamber and statoliths maybe able to do the job as the nerves leading off from them respond to low frequecy vibrations (called infrasounds!).
So I don't think you can mark your student wrong, especially when the scientists can't agree!
Hope this helps!
Hanlon, R. T., Messenger, J. B. 1996. Cephalopod Behaviour. Cambridge University Press. p. 14