From one of the NACS discussion lists comes a referral to Green Earth Book Recyclers.
If you and your students have unwanted books throughout the year, consider partnering with this company. They do not sell books online and provide certificates of destruction for all batches of books sent to the grinder. Other books in good shape, especially children's books, are batched and donated to charitable organizations.
Find out more about Green Earth Book Recyclers on their web site: http://www.greenearthbookrecyclers.com/index.html.
The Retail Muse