Lighten the load!
Carry only those items that are required for the day or find a new lightweight backpack. Organize the contents of the backpack by placing the heaviest items closest to the back. Some students have two sets of books so as not to have to carry the heavy books to and from school. A backpack with wheels can also help if the person has to carry greater than 15% of their bodyweight.
Planning out what books and binders you need for each day can benefit you by leaving some of the overall weight behind for the day and allowing you to have a much lighter load to pack around for the duration of the day.