I had a Toronto artist, Morgan Zigler, do a workshop for my teacher candidates on working with willow in the spring; we had a great time! It’s an easily-renewable, accessible, (often free) natural material to work with, making it just about as sustainable as it gets. We had a lot of fun learning the basics of working with willow, and making a few sculptures for the OISE Learning Garden – a few photos of my students at work are below. Morgan has done a lot of this type of work at Evergreen BrickWorks, modelling how to use hands-on, experiential learning very effectively with youth; he has also done some great installations with using living willow in schoolyards. You can check out his website at:
http://foolishnature.org/homely/environmental/wood/wood.html
If you want to really take this to the zenith with your students, check out the work of Patrick Dougherty; he uses willow and other types of branches to create the most amazing natural architectural works!  His website is beautiful in its own right, and contains lots of photos of his work: http://www.stickwork.net/ Guaranteed to get any learner excited about the possibilities of working with willow!