Did you buy the full (unlocked) version or the version unlocked with in-app purchase? If this was a free app unlocked with an in-app purchase, it should be possible to restore it. You can try going to the Parents’ section (or Settings) in the app and tapping on the Restore button.  

If you have purchased the fully unlocked (school) version, it might be possible that you still run the free version (as they have similar icons).