Your question can best be answered by your local School District Administration office. Try calling or looking online at their website. Start dates vary by states and counties within states in the U.S.
If you don't have the phone number for your child's school or school district handy, and if located in the United States, you can use SchoolFinder to get that contact info: http://www.education.com/schoolfinder
Below are some additional resources you may find helpful for Back to School prep.