Two years ago, a seventh-grader at Carolina Friends School figured out a way to make a difference. After reading a book on global warming, Davi Nydick-Cheshire decided the time had come to do something.   "Up until then I knew that global warming was bad, but I thought it was something I had no control over," she says. "The book taught me about all the little things my family and I could do to reduce our carbon footprint."   Davi emailed Mig Little-Hayes, a teacher at Carolina Friends School, about starting an environmental club at school. A good number of students became interested in her idea that school year. After holding a naming contest, the group officially became the Eco-Chicos, with Mig, the teacher, in charge. The group meets during lunch break once a week to plan environmentally related activities, such as recycling fairs and other events.   Our biggest ongoing project is the selling of reusable grocery bags. They're made from 100 percent recycled materials and are manufactured by the Elizabeth Haub Foundation. They are sturdy, low-cost bags, and they've been quite popular at school. The Elizabeth Haub Foundation helps to develop environmental laws and legislation worldwide. The main reason we sell these bags is to get people to stop using plastic and paper bags from stores. The Eco-Chicos have raised over $1,000 by selling them and hope to buy something big to make the middle school greener. Another project is "Trash-Free Tuesday." We turn all of the trashcans in the middle school upside down and put one trashcan out for people to use. We weigh the trash the day before and at the end of the day on Trash-Free Tuesday. People are encouraged to bring their lunches in reusable containers, and to recycle and compost.   We also collect items that often end up in landfills, such as stackable yogurt cups, juice pouches, cookie wrappers and cell phones, and send them to a company called TerraCycle. It recycles these items, making them into tote bags, backpacks, lunch boxes, folders and other creative products. The yogurt cups are turned into flower pots. TerraCycle sends us collection bags or boxes for sending the items back in, and even pays for the shipping. In return, the Eco-Chicos make a small profit.