And it helps to make your expectations clear for the class and to define what it is they are going to be doing. Get as many people as you can into your classroom to help you and also to serve as role models. The more adults that kids see sticking their hands into the dirt and getting dirty, the more likely they are to do it too. And it's always good to have a diverse range of role models. There are a couple of good ways to find university students who want to work in the classroom.
Some universities have student-run volunteer organizations.
Students sign up, and you can contact them directly. Another way is to contact the Education Department at the university. MIT has a Web site where you can actually fill out a profile telling what your needs are, and then interested students can get in touch with you by E-mail. Another volunteer I work with is a local ranger. She is actually hired by the city of Cambridge, which is unique in Massachusetts.