And most students don't carry their heavy backpacks correctly, with both straps, but instead hang them on one shoulder.
"A lot of the children don't realize how much more stress they're putting on their shoulders," said Dr. Sheila Rao, a pediatrician at L.A. Children's Hospital.
Dr. Rao says young bones are still developing so that puts them at more risk for injury. Over time, carrying a backpack the wrong way can cause nerve and spine damage.
She says the bags should not be that heavy and kids need to strap their backpacks on both shoulders with the bag as high on the back as possible.
"Try to distribute evenly on both shoulders and don't exceed more than maybe 10 percent of your weight. So really, for most school age children, it shouldn't be more than 10 pounds," said Dr. Rao
Parents say that's probably unrealistic.
"I think it's definitely more than that, maybe 20 percent. I don't know," said parent Diane Swik.
Swik says she could barely lift her son's backpack. But he has to use one because for him rolling bags are not an option because they don't fit in the lockers.
So for these kids, Dr. Rao recommends students plan ahead and figure out which books can be left at home on a given day. Or be resourceful like Swik and get duplicate books inexpensively online.
"One was like $7 and the other was like $10 or $15. So he could keep one at home and he didn't have to carry them back and forth," said Swik. "That helped quite a bit."
Doctors say when buying a backpack look for one that fits your child correctly. Look for one with wide straps because they're more comfortable on the shoulders and a waist belt, which can help distribute the weight more evenly.