The Pokémon School introduced in Pokémon the Series: Sun and Moon was a unique concept for the anime. Rather than have Ash travel through the Alola region with a couple of Gym Leaders or Protagonists, Ash spent much of his time attending classes with some Trial Captains and Lillie. It wasn't just two or three companions either; Ash had a record total of five close friends in Alola. For how different the school setting made things, perhaps there's a way the anime could have doubled down on it.
Five friends may be a lot for Ash, but as far as class sizes go, it's really small, even by college standards. The ideal class size would be something closer to 24 or, by Japanese standards, 30. A full class of unique and well-developed characters might be asking too much, but just a few more students could have really sold the school setting. With that in mind, here's a look at potential students for the Pokémon School and how it all could have turned out.
A couple of potential candidates for students would be Elio and Selene, the protagonists of the Pokémon Sun and Moon games. Both of these characters would be around Ash's age, so it would make sense for them to be in the same class as him. They could even have separate goals from Ash's Island Trial like competing in the Battle Royal or collecting Z-Crystals. Ash is apparently supposed to be a stand-in for Elio judging by his clothes, but Selene could still join the cast. Assuming the anime somehow found a way to add these two, that would bring the class size up to eight.
The other Trial Captains could also be considered for candidates. According to the games, Trial Captains can be anywhere between the ages of 11 and 19. Given how Kiawe, Mallow, Lana, and Sophocles are all in attendance at the School, it's reasonable to assume that they're all on the younger side, at least within the anime continuity.
It also doesn't matter what Island the Trial Captains hail from in the games. Kiawe, Mallow, and Lana were all originally from Akala Island. Sophocles was from Ula'ula Island. Despite this, all of them either travel to or live on Melemele Island where the Pokémon School is located. If more Trial captains were to enroll in the Pokémon School, the anime could certainly find a way to make sure they all got in their seats before the bell rang.
Besides the Trial Captains already at the Pokémon School, there's still Ilima, Acerola, and Mina. Ilima is an alumnus of both the anime's Pokémon School and its in-game counterpart, so he's out of the question. Acerola looks a little on the young side, so she could definitely pass for one of Ash's classmates. Mina, on the other hand, would be hard to sell as somebody within Ash's age range; she's apparently closer to the 16-19 age range in the Sun and Moon games, so maybe she could be an upperclassman. With Acerola as the only candidate, that brings Ash's potential class size up to nine.
Another potential candidate would be the Island Kahuna Hapu. It's hard to tell how old she is, but as long as the anime is playing around with ages, she could reasonably be around Ash's age. Assuming she's a worthy candidate, the class size could go up to an even 10.
Hau could have also been a part of Ash's class. For whatever reason, he doesn't show up in the Sun and Moon anime until Episode 97, "No Stone Unturned!" Had he been introduced sooner, he could have been the 11th member of Ash's class. This is about every character from the Sun and Moon games who could reasonably be enrolled in the Pokémon School.
Of course, as nice as it would be to make the class size at least a little bigger, it's important to remember what made the Pokémon School work as it was presented. By keeping the main cast small, more time could be spent developing and making the audience more intimate with the characters. Adding more students may have made the classroom setting more believeable, but it also would have meant less time to get to know any one character. It's better to keep the cast small and familiar rather than bloated and shallow.
With that said, it's possible that a broadened cast could have worked. The other classmates could have simply acted as background characters while the main cast remained what it was. If the other characters needed to be developed, they could have had episodes devoted to them. Until then, keeping them in the background would be like the anime's way of saying that they had everything planned from the start. Expanding on certain characters could have been done either sooner or later and it would have turned out just fine.
As it is, Sun and Moon is good, but maybe it could have been more fun with a larger main cast. At the very least, it would have been nice to see characters like Elio and Selene featured in the anime. For all the kids in Alola around Ash's age, it certainly would have made sense to see them all attend the same school.