User blog comment:FairyRaven/Fairy Tail Chapter 325: Unity, Review and Commentary/@comment-49.176.34.54-20130330054337/@comment-7418318-20130330172631

As I already said before getting sidetracked about how unlikely it is that she's a Dragon Slayer in the first palce:

I think it is possible. If the 80+ barrier triggered for some other reason than Natsus and Gajeels actual age, Hisui might have had her tutorship at a dragon for a few years and then returned to the castle after the dragon disappeared in 777.

One thing I am absolutely sure about is, that the teacher (if ever there was one) is definately NOT Zirconis himself. He hates Humans, except as food, and thus would never teach them anything at all.... least of all how to kill dragons.