Most schools will have a block schedule so i recommend having 2 binders for each day. So if your school has 6 classes and you go to 3 each day you could make a binder for those 3 classes and divide the binder using tabs to reduce the carry weight.
Another tip is, don't put keychains and other things on the outside part of your schoolbag if you don't need them. It will just make your bookbag heavier for no reason. Hand sanitizers are fine because you may not have acess to a sink.
Some things that are not the most important but still useful are things like a mirror, deodarant, and even a portable umbrella if you ride the bus or have to walk home. Also a word of advice bring a flashdrive because you will more than likely have to use a computer and save your work onto a flashdrive. Also something you should definitley have with you is colored pencils or crayons because you have to color more in middle school then you do in elementary school.
Another thing i would reccomend is to split up your pens and pencils. Put half of them in your bookbag and keep half of them at home. It will help you not to loose your pens and pencils.