I found that my HS students even have a very difficult time
seeing the values on real objects, esp. colored objects. I think
it's just too much for them to take in at once. When we begin
drawing after doing value scales, they select a B&W photocopy of
a photograph to draw from. That way, the values are already
"prepared" and they can see the range of values better. Since
there are only 3-4 different photos, I can point out in a short
amount of time where the highlights, etc. are.
When we begin doing figure and portrait drawing, I do the same
thing with photocopies of heads. Then they can better observe
how the various facial features are simply a series of values. I
think students are so trained to outline everything that they
have a hard time making the transition to values. It helps to
use charcoal sticks or Conte crayon so they _can't_ draw lines.
I also point out that our noses don't have outlines around them,
nor our lips and eyes (barring makeup). The reason we see these
features is because of the values around them, both the shadows
and the highlights. It's the values that define the objects, not
outlines. From there, they can go on to color pictures of
themselves, family members, or friends.