I really would get everyone you know to weigh their backpacks, and work on making a presentation about it. Thing is, you all have to make sure that the packs aren't 'loaded' with many extras. Keep the weight legitimate!
Yes, there are alaways a few extras, but they should be kept to a minimum or you will lose credability if you are going to try to make a point with facts. Weigh them and make a list of what's in them, including seperating out the weight of any personal items, like "One small zipper bag for personal items." If the backpacks include too much extra stuff, the PTA will just say that kids carry too much extra 'junk'.
There are things teachers can do to make it less necessary to have all your books dragged home every day. If you are issued laptops, then surely some of it can be loaded on your laptops, or some things can be sent home in handouts instead of using all the books all the time.