The first thing to do is to make sure that the backpack is not overloaded. A good rule of thumb is that a child should not carry more than 15% of their body weight on their back. I realize that many kids have no choice but to carry a lot of books around, but they can offset some of the weight by carrying some books in the arms.
Secondly both straps should be used on a regular backpack. In a large group of kids you can probably count on one hand the kids who use both backpack straps. By using only one shoulder to carry all that weight you do a few things. Instead of distributing the weight on 2 points you concentrate it on one. You also lump all of that stress onto one side of the body. This makes the shoulder and trunk muscles of the opposite side have to work constantly to keep the child from falling to the side while the anterior muscles have to keep them from tipping backwards. For growing muscles and bones this is a recipe for disaster.
The third thing is how the backpack gets from it's resting place to the child's back. The usual maneuver is to grab it with one hand and whip it across your back. This motion puts a lot of torque into the spine and can be very damaging to ligaments and the discs of the back and can easily lead to muscle strain. The best way to put a backpack on is to put it on a table and get into both shoulder straps simultaneously. This prevents the momentum of a heavy bag pulling on the body and it also keeps the backpack from slamming down onto the child's back.
Making frequent trips to the locker during the day and a little mindfulness can make a big difference in the spinal health of children and help prevent injuries now and in the future. A few small changes in youth can save a lot of pain in adulthood! Any further questions on this subject may be directed to the email address below.