Remember the difference between IFR and IMC flying.
You can fly in accordance with IFR in ANY conditions, but to fly in IMC conditions it is in "sole reference to the instruments."
If you are flying a student and you do happen to be in IMC all the time, then you should BOTH log it as so. If however it is a gin clear day and only the student is behind the screens, then they log it as IMC time and you don't, since you are still flying with reference to the outside world and not soley on instruments.