The school nurse needs your medical records. You forgot your gym shorts. Freshmen are running in all directions, looking for their homerooms. If you're headed to a new school, try to arrange a visit the week before classes begin. Explore any areas that are of particular interest, such as the gymnasium, library, or science labs.
Some schools offer maps - get one and give it a read before school starts.
Your first day is also the time to bring in school supplies and paperwork. If so, bring them all with you the first day. Don't forget some cash for book deposits or after-school activities. Wear what makes you feel good. If you simply must wear a new pair of shoes, break them in a few days before or your tootsies may scream for relief long before final period.