Get plenty of notebooks, pens, and pencils for school. If your teachers sent out syllabuses ahead of time, scan each document to find the list of required materials you need to purchase before school starts.
Get your bag ready the night before so you don’t have to rush. If the first day is going to be orientation, you probably don’t need all of your textbooks.
If you have a set uniform in your school, take it out and keep it aside for the morning. If they don't, make sure your clothes aren't violating your school's dress code.
If you need to pay for lunch, get your money the night before. If your school requires you to bring your own lunch, pack it and put it in the fridge before you go to bed.
Your high school likely sent out several emails about the first day. Review any communications from your school regarding the dress code, schedule, and requirements for the first day back.
If you have found your class schedule, review it before the first day. You can also create a cheat sheet listing each class and class number Having a sense for what your class schedule looks like will give you a feeling for what your average day is going to be like.
You may be excited or nervous, but try to get some rest. Aim to get at least 8 hours of sleep before the first day. That way, you'll be refreshed, energized, and ready to go!
Give yourself at least 30 extra minutes in the morning to eat, shower, and get ready. A little extra time will ease things up and put you in a better headspace on the first day of school.