I've heard that some schools are getting rid of the school lockers. Rather silly idea if you ask me.
As far as the backpack. When in school, I used to carry a duffel bag back and forth to school that was absolutely monstrous. But in reality I rarely looked at everything that I was carrying.
I'd encourage going through the backpack one evening and try to figure out what is needed, and what is not needed.
Perhaps it means doing a bit of scheduling and planning. So, what day does one want to do social studies reading? And what day does one want to do math?
Just carry the books home that one actually needs rather than lugging everything back and forth every day.