It's certainly not good for children to be carrying to school more than one-fifth of their weight over the shoulders and on their backs. Backpacks can be awfully heavy, when one factors in all the textbooks, notebooks, and school supplies. Much of the weight is the classroom-related material, but of course, some kids also bring sports equipment or musical instruments, adding pounds to an already heavy load.
Here are a few things you can do to try to make it a bit easier (lighter) and decrease the risk of injury for your kids. Have them avoid carrying anything that is more than 20 percent of their body weight. For example, if your child weighs 50 pounds, she shouldn't be carrying more than about 10 pounds. Sometimes, this seems impossible to do. Help your child sort through everything before packing up and see what can be left home that day. Place heaviest items in first; the closer they are to a child's back, the less strain they'll put on those muscles.
Buy an appropriate-size backpack, one that ends just a few inches above the waist. Use a backpack that has soft, padded straps to make it a bit more comfortable to carry. I would encourage your childen to carry their backpacks over both shoulders, rather than to one side, in order to better distribute the weight. Also, make sure the kids are bending their knees when they first lift the backpack, to avoid further strain on their back muscles.