Get your school to adopt and enforce a Code of Conduct that ensures that the products it buys are not made in sweatshops or by child labor. The information from Minnesota and New York will help you get started. The United Students Against Sweatshops, USAS, has been very active in the fight against sweatshops. You can also get in contact with local USAS members and work together. Find out if any groups in your area are working against sweatshops.
Local unions are a great place to start.
Even if they are not working on the issue, they may be interested in learning about sweatshops and helping you out. Ask your parents if they are members of a union or if they know anyone who is.