Used to teach quilting/patchwork to many ages. All suggestions excellent-but a few more ? How old is this student? Is this a first project? How are you going "quilt" or secure layers together? (sheets are hard to quilt especially by hand) If you are using "tying" then new sheets work. One thing we used to do is ask for fabric for top so student can start sewing and " contract " for backing from parent when top is complete. You might also provide list of reasonably priced sources. In ou area that would be Walmart, and Joannes during a sale or even "goodwill" if they understand what kind of fabric you need for project. if you get stuck start asking around -many people buy fabric and never use it.They are only too willing to donate and support a child. Another place to appeal to is your local quilt group and their friends . A quick thank you note delights many people and builds community for kids. It's also possible your local quilt shop has a "dog" bolt they will donate or sell for very little-one class of teenagers I worked with took donated bolt and everyone of them used it in either the front or on the back. One person's ugly became part of a very successful class.
In this economy $35 or $40 is a lot of money for people to invest especially if they don't sew themselves