Americans generally get driven to school. The backpack is worn only between classes at school and then moved from car trunk to home, not worn during those times.
I personally wore out a $100 Eddie Bauer backpack every other year or so during middle/high school. I walked to school every day for over a mile, rain or shine. The zippers would completely fail, or the shoulder straps would detach. Eddie Bauer had a lifetime warranty so they replaced the backpacks without question.