On your first day of school, it is important to remember that you are going to be going through a day where you will be on your feet and be doing things that you are not used to. For example, you are used to sleeping late during the summers and laying around on the couch, but now you are required to sit in a seat, learn, interact with other students, walk to different classes and have recess. Your dress or attire needs to be considered about the events that are going to be encountered during this day.
Once you get used to it, then the dress can be different. However, on the first day, you should dress both nicely, cleanly and comfortably in order to endure the entire day.