Shoulder muscles become tense and sore, and may even have long term damage of sensitivity, or could even lead to a problem with scoliosis. I used to carry my backpack slung over one shoulder when I was in a hurry to get to my next class. It caught up with me. When choosing a backpack for the upcoming school year, take into account what fits you comfortably. Keep the tags in case you have problems with it, so that you can return the backpack.
Comfort is more important than style or looking good.
It will all add up in the end. Although backpacks slung over one shoulder may be fashionable, this posture puts further stress on the back and spinal cord, causing more back problems later on in life.