Fortuitously, I came across this recent article in Archives of Disease in Children on my daily review of medical titles.
In an analysis of 1403 children, 61 % were carrying backpacks that exceeded 10% of their body weight. This was significantly associated with back pain but not back pathology. The long term effects of heavy backpacks is not known, however, I suspect that it is not significant.
Assuming that the packs are filled with books, perhaps as the use of i-Pads and e-readers proliferate, the heavy textbooks will take a digital format and lighten the load.
As for me, I will pass on the backpack, and opt for the over-the-shoulder 'man bag'.