First, you didn't mention what size pack you are looking for. You may think saying daypack/overnight pack defines it, but it does not. For an overnight what gear are you bringing? If you can't answer that, then it is hard to get the right pack to carry it.

Second, like much of dual-purpose I think you will find this does both tasks poorly. There tends to be a large density difference between backpacking gear and school books. School books are heavy and you usually do not carry them very far. As a result most people get a pretty small pack with limited suspension. Suspension might be nice, you think, but do you want to clip in and out every time you sit down with while walking around campus? Backpacking gear is less dense, so even though it may weigh the same you would need a larger pack. People tend to carry it for longer periods of time too. That means suspension is more important