I just finished that bag! :D Its in different colors, dark brown with light blue stripes and handles. It was really easy and turned out cute! I'm not sure if it would be durable enough or quite big enough for a book bag. Its more purse sized. I'm debating whether to line it our not. I think I might put something in the bottom to make it a little more stable. The bottom does tend to droop a little when it is picked up.
Fiber Trends has a pattern that would work for a book bag. It is AC-43 Fabulous Felt Totes. I've made two of them in the medium size and they are quite large. The first time I used Brown Sheep yarn and the second time I used Cascade 220 and I really like the Cascade 220 better. It felts more quickly and seems a little sturdier once felted. Check out fibertrends.com and look under accessories.