Public school students are the victims of compressed teacher work schedules and of increased homework loads. In order to reduce the length of teacher work days while continuing to deliver mandated classroom time minima, the average time periods between classes have generally become shorter than such breaks in past decades. Students no longer have enough hall time to simultaneously swap books in and out of lockers and avoid tardy slips. Thus, the larger backpacks and rolling backpacks on campus. Increased homework speaks for itself.
BSA guidelines for backpack weight are, I believe, a maximum of 15% of body weight. I believe that the majority of school children (in my local district) routinely exceed these guidelines.
Increased use of technology such as kindles (ebook readers) and Internet-based textbooks would help reduce these developing back injuries. Of course, technology can hurt, too (e.g., increased use of laptop computers that are backpacked).
See http://www.icefonline.org/information.asp for more information and some cites.