Look for a daypack with an internal frame. Do a search on Amazon for an “Internal Frame Backpacks.” Check models like the Kelty Redwing series or find one on wheels if your teenager isn’t too humiliated.
Using an internal frame with a waist strap makes the pack sit on your hips instead of putting pressure on your shoulders. The chest strap holds it close so it doesn’t hang outward and distort your kid’s posture.
If you have to shrug your shoulders to keep your backpack from slipping or lean forward to balance the weight, you are putting stress on all of your neck, shoulder and back muscles.
Check out this Kelty pack as an example. Look at the alternative views and specifications. http://www.amazon.com/Redwing-3100-Internal-Frame-Backpack/dp/B001PACCE6/ref=sr_1_36?ie=UTF8&s=sporting-goods&qid=1245617979&sr=1-36