So, Samson Oak is able to pull some strings to get all of the Champions to come to the Pokemon School (Ash was probably involved but shh). The Orange Islands & The Battle Frontier also send representatives (incl. Ash). They all have little decorated platforms outside to present while they talk about their region and stuff. They go in the order Kanto -> Johto -> Hoenn -> Sinnoh -> Unova (Iris or Alder or Both, your choice) -> Kalos -> Orange Islands -> Battle Frontier. The only champion who actually knew about Ash being in Alola was Lance.

Ash has to do something with the Kanto platform, who knows what, could be something as minor as having to use Pikachu's electricity to light something up. When they get to the Orange Islands one, everyone's kinda disappointed at first because "Wow, their champion didn't come, and it's not even like they're a whole region anyways. All of the major Champions showed up even." Ash probably then does something and makes it 10x better by revealing he's the champion. He also makes another cameo on the Battle Frontier one because why not?