Is your backpack too heavy? If you did not carry the pack during the summer and did not do some athletic activities to keep your back muscles strong, now that you are back in school, you may be carrying more weight than you can handle.
Especially since it is okay in the morning, then gets worse as the day goes on. Also the number of books you have to carry around this year, may be more than you used to.
You can try carrying a couple of your books in your arms, rather than placing all of them in the pack. It will balance out the load, and take some of the weight off your back muscles. Also, if you don't have to carry a book, leave it in your locker. With some time, you will build up the muscles, just like an athlete getting into shape.
But, if the pain does not get better in a little while, or it gets worse, spreads to other areas, or you develop other symptoms, you may need to talk to the school nurse or your physician.