Backpacks are considered the most efficient way to carry books and other items needed for school. Besides, they are fashionable, hold more items and come with multiple compartments to keep them. Backpacks are considered safer because they distribute the weight evenly across the body and are supported by the back and abdominal muscles. 
However, school age children are in a critical development stage regarding musculoskeletal development. Spinal ligaments and muscles are not fully developed until after the 16th year of life. Students represent the largest group of individuals that use backpack from of load carriage, with the daily responsibility of transporting a variety of items to, from and around the school. There has been a growing concern about what damage are these backpacks doing to their backs, mainly the combined effects of heavy loads, position of the load on the body, size and shape of the load, load distribution, and time spent carrying. 
The American Pediatric Association recommends that students carry no more than 15% of their body weight in backpacks (55% of students carry more than the recommended, and 60% of the children’s orthopedic visits for back and shoulder pain are the result of carrying hevy backpacks).  However, in a study performed by Chansirinukor et al, a significant reduction in the craniovertebral angle (increased forward head position) was found whilst carrying a backpack weighing 15% of body weight over both shoulders. This implies that the weight of the backpack has an effect on changes in cervical and shoulder posture, suggesting that carrying a backpack weighing 15% of body weight would be too heavy for high school students aged 13 to 16 years old to be able to maintain their normal postural alignment. In other words, carrying a load of less than 15% of body weight could be recommended 
Why heavy backpacks are dangerous?
When a backpack is filled with heavy books and incorrectly positioned, the weight’s force can pull your child backward. To compensate, your child may bend forward at the hips or arch his back, causing the spine to compress unnaturally. This can lead to shoulder, neck and back pain. Wearing a backpack on one shoulder can cause your child to lean to one side to compensate for the extra weight and can also lead to pain. Undue downward pressure is put on the top of the shoulder and then weight is then dispersed unevenly down to the hip, knee and ankle. 
Girls and younger children may be especially at risk for backpack related injuries because they are smaller and often carry loads that are heavier in proportion to their body weight. 
Choosing a right backpack
Choosing the right size backpack is the most important step.
The airpacks system backpacks feature a patented ergonomic design that redistributes weight and reduces stress on the back by up to 80%, lightening the effective load by 50%. Also, they have an inflatable lumbar cushion and padded shoulder straps. They also have wide, foam-cushioned shoulder straps. It is sized by the wearer’s height to ensure good fit and prevent overloading.  Also, backpacks should have a hip and chest belts to transfer some of the backpack weight from the back and shoulders to the hips and torso. 
If the backpack is too large it will sit too low, placing undue stress on the spine and spinal muscles. The larger the volume of the pack, the more likely it is to be overloaded. Keep a backpack use to necessities only. Clean out your backpack daily, removing items that can be left at home or in a locker. Distribute the weight evenly; a backpack with individualized compartments will help position the contents effectively to spread out the weight. Wear both shoulder straps, snug but not too tight; this will help to distribute the weight evenly across the back and promote good posture. 
If you are forced to move forward to carry the back pack, to slouch or to lean to one side that means that the backpack is overloaded.  Change in posture when wearing the backpack, struggling when putting on or taking off the backpack, pain when wearing the backpack, tingling or numbness in arms and legs, mostly arms, or red marks on the shoulders, are also warning signs that the backpack is too heavy. 
Sitting in school
Students spend a good proportion of their time at school in a chair. Good sitting posture and furniture of the appropriate size is essential for the student’s successful participation. Good sitting posture will prepare the student for the physical and cognitive demands of class work and will also reduce postural fatigue. The pelvis should be symmetrical and upright with bottom well back in the seat; the hips should be flexed (bent) at 90 degrees; the spine should de symmetrical, preserving the normal curves; the head should be upright and symmetrical, not leaning over the desk; the knees should be flexed at 90 degrees, with the thighs well supported in the seat; the feet should be at 90 degrees to shin and flat on the floor. Concerning the height of the desk, the students should be able to rest their elbows comfortably on the desk at approximately 90 degrees. 
 Cerebral Palsy Alliance. Sitting Posture. Early school years fact pack; 9:32-33
 Illinois State Board of Education (2006). Carrying Backpacks: Physical Effects.
 Ibrahim, A., El Kahky, A., Al Sayeed, H. (2006). Comparative study of static and dynamic balance of school age children with and without backpack. Bulletin of Faculty of Physical Therapy, Cairo; 11 (2)
 Fitzpatrick Physical Therapy (2007). Healthy “Back” to School. Vol 1 (3)
 Chansirinukor, W., Wilson, D., Grimmer, K., Dansie, B. (2001). Effects of backpacks on sudents: Measurement of cervical and shoulder posture. Australian Journal of Physiotherapy; 47: 110-116