Well, I was hoping for more leaks or spoilers for this episode. Seems like some fans are speculatating Ash attending academic studies in Paldea to improve his knowledge of Pokemon battling.
Correct me if I'm wrong, but in the Pokemon World, a licensed pokemon trainer, which usually is given at the age of 10, is considered eligible to be a professional trainer, as they are given a starter and allowed to compete in gym challenges and tournaments since they are registered in a official pokemon trainer system.
I thought the anime had long ago covered the topic of why Ash doesn't attend a Pokemon training school a while back in the original series, such as in the episode "School of Hardknocks" and the episode that featured pokemon examinations to get into the Pokemon League.
It was made clear that Ash prefers to travel and learn the skills of being a professional pokemon trainer through the contemporary means of traveling to many regions and participating in gym challenges and tournaments.
The Alola school arc, is slightly different, since Ash was just attending a local school where he could learn more about the culture of the Alola region. The Alola school he attended was not structured to be a Pokemon Training school where locals could go to get some form of certification to compete in a official pokemon league, like what was featured in the Pokemon training schools in the original series.
Also, as others have mentioned, Ash is a professional trainer, it narratively does not seem proper for him to back track and attend a beginner or intermediate pokemon training school when he's already classified as a licensed pokemon trainer that can compete in gym challenges and regional tournaments. However, if Paldea suddenly introduced a high level pokemon academic program where elite trainers could get a certification or diploma that would classify them as a Pokemon Master, then that would seem ideal.
Some fans even speculate Ash to be an instructor, and that doesn't seem plausible, since Ash is not a certified academic research professor, like Professor Oak, and we've seen many time in the series that Ash is terrible at his dialogue of explaining to people how pokemon battles work.
If this episode does reveal Ash's decision to visit Paldea it will be to take on some high level challenge, such as catching a legendary or battling in some type of tournament.