Dad jokes are great. Even if Ardbert's technically the "Other Mom" to Hythlo.

Ardbert looks over to Noctis in genuine surprise. "School lasts that long where you are? Wow. Is it that way for everyone? What sort of things do they teach you?"

He's since learned that Noctis is a prince, and being a ruler means you have to know a lot of things. It wouldn't be surprising to hear Noctis would have to stay in school longer than the average person. But he's also not going to bring up the prince thing because Noctis wants to be seen as an ordinary guy, just like his granduncle.