My rule of thumb for still life is to let them use viewfinders and only
do a part of the still life. In that way they can
concentrate on the part that interests them.
I also have my students use viewfinders. Another thing I do with
still-life, is to set up three or four different groupings with different
type objects in each one, so students have a choice of drawing things
that interest them the most. My room is really small, but I put several
of them on rolling carts--and student use drawing boards and move their
chairs to find a good location. (this is for elementary, incidentally).