Sometimes is difficult to explain how many tonal value you can find in a black & white pictures because the scale of lights and shadows can be very wide. I’ve found that the students can understand better if they try to copy a photograph or a work of art on a tracing paper placed on the original picture.
It’s a trick, obviously, but the student uses a different colored pencil (not black) to understand which is the right hand pressure to obtain the gradient. And the result is impressive!
Have a look even at my old post about Romanesque bas-relief.