September 13, 2010 
I think the easy answer here is YES! You should go weigh your kid’s backpack. The experts say it shouldn’t weigh more than 10-15% of their weight. So, if my kid weighs 80 lbs, they can only carry 8-12 lbs of books. With the average textbook weighing 3-4 lbs, that’s not much.
Studies at the Mayo Clinic and at other hospitals around the United States calculate that overpacked or improperly packed school backpacks account for more than 7,000 emergency room visits annually. And according to the American Occupational Therapy Association, over 50 percent of all students between the ages of 9 and 20 suffer from chronic back pain related to the way they wear their packs. (source)
So, what do you do? I’d like to say that you don’t bring all you books back and forth and put everything online, but I don’t think that’s going to happen anytime soon. A logical answer is that you give the kids bags with wheels that they pull around. That seems easy enough.
A few other links on this topic which include other points:
Enabling Healthy Decisions 
This is a real issue that is worthy of consideration. While looking further I uncovered this article that addresses some of these concerns: http://www.overstock.com/guides/backpack-buying-guide