My suggestion, having just done a very introductory unit on sumi-e, would be to have them practice (and practice and practice) some of the basic strokes before attempting to do compositions. I made a little packet for my kids (12 to 17 years old), in which they had to emulate to the best of their ability the individual and combined strokes shown. At the end of each section was a small composition for them to copy. Only when they had done the entire packet could they go on to create 4 small (4"x6") compositions on the good rice paper. We then made name seals by carving gum erasers and printed them with red acrylic, taking the composition into consideration. I really stressed the importance of using a variety of grays, as well as pure black, to achieve the illusion of color. We also discussed the importance of nature imagery, ie insects, birds, and plants. Another focus was on using ONE quick brushstroke to create each line or shape (they really had a tendency to want to "sketch" the forms with lots of little marks; they also wanted to outline and fill in rather than "build" the form.)
I had kind of expected a lot of resistance towards all the practicing, but was surprised by how much my mainly inner-city kids seemed to take the practice seriously and really enjoy the process (and finished product! I had one kid shouting, "I'm a genius! I'm a genius!" when he figured out a way to do pine trees!)
Good luck - I hope this helps!
-Lydia de Toledo