Discovery Education's Turfmutt is another great, free resource for teachers from Discovery Education.
Turfmutt has lesson plans, videos, puzzles, activities, resources and more for educators and students to learn about the environment and how to protect it. There is even a contest for classes to enter.
Most of it is geared towards grades 3-5, but I found plenty of material and activities that are good for middle school and high school.
If you teach environmental science or about the environment, this is a resource you need to check out.