Our 12 year old has complained since 5th grade of neck pain, and now he's added back and leg pain to the list. We've reduced the weight of his bookbag several times, but it's still too heavy. This weekend I'll need to go through it with him again and see what else can be eliminated. The single biggest item is a large zipper binder which the school insists each child carry to schlep around paperwork and school supplies. They can only go to their lockers at the beginning and end of the school day and at lunch, so they continue to carry a lot around all day.
Our son is a sturdy kid - competitive swimmer and in all other respects healthy and of normal size. He's neither puny nor unfit. Kids today are simply being asked to carry more than is healthy for them, in part because of the huge homework load. One good thing is that some of his teachers keep a second set of books at school so that the child can leave the text book at home. Alternatively, some use an on-line version of the text to support homework.