First,
I have flown a number of types (and indeed teach on a type) where that is simply not the case.
This is not the case on some aircraft I believe before the 70s but I do not hold me to that one I would have to look that up. But newer aircraft, all new Cessna and new pipers use enough for it to be visible to the naked eye. Some older models may have a small degree of difference but it might go unnoticed if not measured.
EDIT: I direct you here:
Adverse yaw - Wikipedia, the free encyclopedia for further information on differential ailerons.
On the other note, I am advocating that as you go through the lessons you introduce pertinent aerodynamic principles on the ground that occurs in the maneuver. When you teach a stall spend 30 minutes on the ground explaining how it is happening and why. Same with weight and balance and talks of CG you can bring up the model from my original post. Simply broaden your student’s horizon for each beginning lesson, that way they truly understand their aircraft.