Except the reveal says absolutely nothing about him "setting up a league", or even adventuring or taking on the Island Trials. The major premise, based on what was said, is he goes to Alola with no purpose other than to attend school. His goal is to graduate, not to build Pokemon Leagues, not to challenge the Trial Captains, none of it.
Just graduating. "Like never before", whatever the heck that's supposed to mean. If he were planning to become the best trainer in the Islands and take on the trials or set up a league, you would think ONE of those would have been mentioned with the reveal in some form. But all we get is, "He's going to school. Oh, btw, he gets a Z-Ring lol but he's going to school."
So suddenly the anime becomes slice of life. :P How they think this is going to help the series, I have no idea, but I have no plans on wasting my life watching Ash go through middle school. Middle school was enough of a nightmare in real life.