Pond succession is a fascinating topic for middle-school students. This video follows the succession of a pond as it changes over time into a forest. Here is great visual you may want to use along with the lesson.
The EPA has a site devoted to wetlands education. Lessons, videos, activities that you can use with your class.
The U.S. Fish and Wildlife Service has a Wetlands Coloring Book with a fun look at wetlands just for you! While coloring these pictures or just looking at them you can learn about wetlands and the animals and plants that live in wetlands.
Wetlands Lesson Units
These four curriculum guides from the Washington State Department of Ecology are comprehensive units that cover many different grade levels. Each section is focused on specific skills and wetland information. Check out all four!!
Discover Wetlands — lessons, activities, experiments — just about everything you could possibly need, except the water.