@AoYokai: Great to see you around again! How've you been?
... Actually that is a very good point. Wasn't it implied that studying abroad was what let Safu see how closed up No. 6 really was? Hm. It would make sense if it was part of the Babylon Treaty and No. 6 is just going along with it in order not to seem suspicious. If all the other cities have exchanges to share knowledge and what not and this one city refuses, they might wonder what else it might refuse.
Then again, it could be a recent thing or even an experiment. No. 6 is still really young when the story starts, and I wonder how long the school system as Shion and Safu experience it has been in place anyway. I also wonder why No. 6 has an airport to begin with (and to the public at least, took down the Mao forest just to build that airport, so it was apparently important to get one). No. 6 isn't that big that you'd need a plane to navigate it. Any trips would logically be going abroad. (And that raises tons of new questions about how safe it is to travel in this post-apocalyptic world, and what you can see from the plane if the skies are clear. Beyond mentions how not the whole world is dead, surely you'd be able to see that at some point from the plane? But okay, I'm already going off-topic now.)
On the other hand, it could be truly as simple as 'making the future elites get as much information and knowledge as possible so we can have all intelligence together'. No. 6 is the most advanced of the six remaining cities, and if it can find a way to make its elites absorb the knowledge that the other cities have, it'd be all-round and self-sufficient and it could close its borders for good. I mean, just because No. 6 is the most advanced doesn't mean the other cities have nothing to offer. Maybe they're just betting on it that they've brainwashed their future elites enough not to let any ~open minded~ education abroad get through them.
Iirc, the goal of the Babylon treaty was also to share knowledge, but now I'm heavily starting to doubt myself... it was definitely to make peace agreements though. But if all cities are open about their own developments, they can also keep each other in check in case one develops new techniques that could be used in war. And being open was part of the treaty, I think?
Note to self: reread, for real, before you spout more nonsense.