Nebula of @apocryphall asked while we were plotting, so here's a more complete headcanon, as long as its in my brain.
After he was discovered in Area Zero by [AI] Sada, Nico was handed off to the Pokémon League for a formal investigation into who he was and how he got down there. Officially speaking, he was in the league custody for about two to three months while they tried to figure out how the fuck this teenager made it to the Area Zero depths on his own. It probably wouldn't have taken quite so long if Nico had been more cooperative about it, but... I mean. He has plenty of reasons not to trust a bunch of random authority figures with unclear motives.
Once it came to light that he was (probably) a long-lost member of the original Area Zero Expedition Team, however, and not some kind of deep crater horror, the question became, "well, now what?" Suddenly there is a guy who, for all intents and purposes, died 200 years ago. A kid. There were some serious questions regarding what to do, where he would go, how he would adapt to modern life—questions that he didn't get a lot of say in, much to his frustration.
Eventually, it was decided that Nico should attend Naranja Academy as a student. He spent the next several months taking whatever lessons were necessary to get him up to speed to function in a modern school setting so he could enroll at the beginning of the next school year. Less work than you'd probably expect, but most of Nico's time for those few months was still dedicated to those lessons. Clavell assumed responsibility for Nico's well-being as long as he's attending school there, but legally speaking, until he's an adult, Nico is still a ward of the Paldea region.