Since I have an attention span of a gnat, I can understand why a full
still life would put kids off. I set up a full still life as well, but
give them view finders to isolate a certain section of the still life.
They then must do the still life they see in the view finder on their
page making sure it touches all four sides of their paper. Then they
render the still life in values. In this way, they are not overwhelmed
with an entire still life, can choose what appeals to them, learn about
expanding their images, as compared to plopping them in the middle of
the page, and work on rendering values. Sometimes, I have them do 6
little thumbnails of the still life, still using the view finder,
emplasizing values again, and then I take the still life down, and they
must do a monochromatic painting based on their roughs. They actually
like that exercise.
Sometimes in our quest to make students better in drawing, we suck the
joy out of drawing, by sheer exercise. I tell kids that drawing is like
exercising, and the more you do, the better you can get, but at the same
time it is not necessary to make it drudgery. I know what I exercise, I
get bored with the same thing, so I "cross train" and thus it must be so
with drawing exercises. (Of course my husband and students are laughing
at this point, because it has been years since I have actually
exercised, but I think you get the metaphor).