Unless you're carrying enough in the pack to keep the bladder in position you'll have to sewn in a panel to keep it from just sloshing around in the main compartment. I don't know what sort of padding the book bag has, but if it's just canvas of some sort you'll be putting the bladder nearly directly against your back, that gets pretty hot and sweaty. Lots of advantages to just buying a good pack with built-in features. I've never done much to modify packs except stitch them together when they get busted up, maybe other people who DIY will feel differently about this.

JimmyTH