The thing that I do with them is talk about " The five principles of shading."
Highlight, Shade, Shadow, Reflected light and Backshading. When they are
doing the shades and reflected light they are working inside the line of the
form, but as soon as they create a back shade around the highlighted area on
the form the line disappears and the edge of the form is produced.
When we critique these, this point about outlines is stressed and they soon
see how the edge of the form is created with backshading. they can also use
the outline of the form and shade a short gradation to create the edge inside
the contour of the form.
For the rest of the years we refer to the 5 principles of shading and they
start to see that outlines are not necessary to show edges of objects.