What should I pack in my high school bag?

The school would normally tell you what you need to bring, but generally you would take: 2 pens and pencils, a notepad, ruler, calculator, drink, food, deodorant (you never know how useful it can be XD), and you may also need a USB storage device, aswell as a PE kit on some days. You might want take some colouring pencils/pens and another drawing pad for doodling.

You will probably not be starting any lessons in any of your classes on the first day. Teachers normally go over the rules, some basics, a class overview, and what supplies you will need.

It is good you are thinking ahead and some of the supplies listed above are correct, but you will also need lots of binder paper and a binder. In the high school classes I teach I find students come without the proper supplies. Usually at the start of each year a list is put out by the schools and sent to stores. One of the things I notice is the lack of organization by students so get folders for each class to keep assignments in and directions for assignments. A calendar will also help you for class assignments and due dates. If you get organized early you will be doing what you need to be doing.