Backpacks have a tendency to quickly fill up with books -- heavy books. So how do you avoid straining your back when everything you're carrying feels like a necessity? Your education may feel like a burden, but that's a lot of weight to (literally) have on your shoulders.
You're Gonna Carry That Weight, So Distribute It Evenly
First things first: when you're packing your bag, you should strive for a fairly even horizontal weight distribution so that it doesn't strain one side of your body more than the other. Heavier items should be on the bottom of the pack; that shifts their weight to your back instead of your shoulders and neck. If you're carrying a lot of books, the heaviest textbooks should be placed closest to your back for the same reason -- their weight will rest against your back instead of hanging from your shoulders.
Wear both straps. It's obvious, but I definitely spent most of my teenage years one-strappin' it for whatever foolish reason. Wearing both straps helps to evenly distribute the weight and reduce the risk of injury. Of course backpacks with wider, cushioned straps will put less strain atop your shoulders than thin straps, so if you have a choice, strive for large and plush.
Some larger bags also have sternum straps that go across your chest to help distribute the load. Seriously: these make a difference. Use them. If your bag doesn't have one, you can pretty easily add a sternum strap to any bag for better support. Larger packs have belts near the waist that help to shift weight to your hips -- but you probably won't be wearing a bag that large for your daily work.
Keep the bag close to your body. If the straps are loose, the bag will swing while you walk, and that actually increases the strain put on your back muscles.
One commonly cited study recommends that a child's backpack should remain lower than fifteen per cent of their body weight. If you're trying to figure out the limit of what you or your child should carry, here's a simple calculator. Even if it's safe for adults to carry more than that (backpackers on a camping trip might aim for closer to 20 per cent), 15 per cent of your weight is still a reasonable upper limit for adults carrying a heavy day-to-day bag.
Stay Organised So You Aren't Lugging Dead Weight
Treating your backpack like a bottomless pit and carrying all your possessions with you all of the time will just make you work harder. Instead, take the time to decide what you need before you start your day so that you only carry the necessities. That's especially pertinent when you're carrying multiple textbooks to classes that you aren't necessarily attending that day.
There's plenty of other clutter than can accumulate too, so you regularly audit your bag to see what weight you can shed. You probably don't need to carry every exam and paper from last semester with you. Kids can be especially disorganized with crumpled old homework stuffed in their backpacks and might need a helping hand to clean them out. It may not seem like a lot compared to those big heavy textbooks, but those papers can add up -- and every little bit helps.
Consider the Alternatives
Yes, I'm talking about ebooks. The adoption of digital textbooks has been fairly slow for a variety of reasons, but they offer an almost literally weightless alternative if you have a tablet or computer that you can use to read them. Even if you stick with paper for the important textbooks, relying on digital alternatives for the books you won't necessarily need all the time is an easy way to shed a few pounds.
You could even scan the passages you need. Laborious, sure, but if you're only going to focus on a specific part of a book for weeks, it might be worth it.
Lastly, you might want to consider not wearing a backpack at all. There's a wide variety of rolling bags that are adaptable -- you can wear them or roll them, and they're usually designed for air travel. If it's really necessary for you to carry a ton of textbooks with you all day, rolling cases are worth considering. There are plenty of child sized versions too.
As long as you keep your travelling library streamlined and keep your pack fitted properly so it's not a pendulum of books, you should be able to avoid any major fatigue or strain. Then you can focus on holding up the more important academic pressure.