I taught HS, and believe it or not, many kids just didn't get it.
I'd hold up a stool with open rungs at the start of each day with
negative space as a lesson objective. I'd have some kid come up and put
their hand thru the open areas and tell others where all of them were.
They had to identify ALL the open areas. It worked like a charm. I
used flat art work in a different manner. I took acetate and covered a
famous piece of Artwork and posted it on the cabinet. I would again
choose a different student to use an oveerhead marker to identify the
negative spaces. I'd change the work and repeat the exercise. The kids
got it really well. Even the LD ED kids were successful.
Hope this helps,