When an app is purchased, it is tied to the Apple ID that was used to purchased it. For personal use, you can then install that app on up to 10 devices that have the same Apple ID. You can also share apps among family members with the family sharing option.

In a school setting, it's a different scenario - you shouldn't be sharing the same Apple ID among multiple devices, and family sharing is meant for personal use, not educational. To deploy to multiple iPads in a classroom, purchase Book Creator via Apple School Manager.

When it comes to using Book Creator (or any other app) on multiple devices in the classroom, the best place to start is with Apple's Education Deployment Guide.