The National Wildlife Federation Schoolyard Habitat Program is just the place to begin your project. Many teachers have already gone through the process and you can get some very good advice. Among the considerations when planning a school project is to think about the garden over the long term and that's where careful planning in the early stages will assure its success.
Once you are ready to think about plant choices make sure that you select native plants that are appropriate to your area. There are many plants that thrive in natural areas with shade and clay soils. Learn about the North Carolina plants by consulting the 'native plant' section of our eNature Online Field Guide. Another search tool is the eNature ZipGuide. Both features will give you a starting point in your plant search. Next take it to the local level and talk to native plant experts about plants for shade; they can be found at area parks, refuges, or other natural areas. Consult with the North Carolina Native Plant Society.
In other words, learn about creating habitat, and that involves a good deal more than creating a garden in the typical sense. It means that you will look at the site's growing conditions (soil, light, air circulation, etc.) and the kind of habitat that can thrive there. It's a great exercise in creating a space that truly adds to the well being of wildlife and the people who observe and care for it. Please note that butterflies will not be attracted to shady places unless seeking shelter. However, you will have no trouble attracting birds to a wooded site with various levels of cover and nesting sites.
Be sure to check out the Access Nature curriculum developed especially for students with disabilities. We wish you well as you set out on this exciting venture!