I've tried to put together some green finds I've found just searching over the internet to hopefully help you make greener choices.
Terracycle Recycled Drinkbox Bookbag. For only $14.99 that's a comparable price for a bookbag.
Bazura makes a great lunch bag with a similar design, also made out of drink boxes. They are made from a co-op of women in the phillipines, and are fair trade. At $15.95 they are also something worth considering.
Hemp Backpack from the Hemp sisters. Retailing for only $47.95, it's a little more of a stretch, but in my mind not out of reach. Definitely something to consider.
For those of you not familiar with hemp, it is a plant closely related to cannibus, but lacking in the chemical to get you high. So don't worry about your teen trying to smoke the backpack, it won't work. Sadly hemp is illegal in the U.S. and most of the hemp here comes from Canada.
Hope this helps you getting started on your back to school adventures with the kiddos. Green can be affordable if you look hard enough.