I think you should petition with other parents, the school your kids go to (if they don't have lockers), and possibly hire an attorney to send a strongly worded letter to the principal. If that doesn't work, contact a local reporter & have them do a story in front of the school to bring attention to weight of backpacks & concern over future medical problems.
Check out this website:
You are obviously not the only one who has worried about it & are smart to be concerned..
Hope this helps!
Coldwell Banker Residential Brokerage
Sherman Oaks, CA