Ilima is a graduate of the school and still visits regularly, where he is admired as a hero among the students and is sometimes called "Prince of the Trainers' School." After becoming Champion, the player can challenge the school's principal, Asuka. In Ultra Sun and Ultra Moon, they can also investigate a series of mysteries if they visit the school at night after defeating Hala and hearing a rumor from a Youngster on Big Wave Beach.
If the player visits Ilima in his house in Hau'oli City after defeating Hala in the Melemele grand trial and agrees to battle him, he will escort the player to the Trainers' School battlefield and battle them there.
In Pokémon Ultra Sun and Ultra Moon, when the player speaks to a Youngster on Big Wave Beach, he will tell them about a rumor about the Trainer's School. After speaking to the Youngster and completing Hala's grand trial, if the player visits the Trainers' School at night, the music will be noticeably different and the area will be empty, except for a single Preschooler and a Drifloon next to the sign in front of the building (the principal's door is locked). When talking to the girl, she will tell the player to solve the seven mysteries; however, she only offers the player six mysteries to solve, saying that she doesn't know what the seventh mystery is.
The player can trigger each mystery by choosing to investigate it by talking to the Preschooler near the sign at the entrance, then meeting the trigger condition. The player cannot trigger an event unless they have chosen to investigate it.
"At night. Near burning place. Scary lights show up and wander. Burning. Someone burning and crying. You hear girl's voice."
Approach the main entrance, then the incinerator
Youth Athlete Deedra is using the incinerator to burn love letters she wrote to a crush who is now in a relationship with someone else. She decided to burn them late at night so nobody would notice. When the player notices her doing this, she challenges them to a battle.
"At night, you hear strange sound. Slimy, slimy—getting close. Footsteps of Lord Slimy... If you're found, Lord Slimy swallow you. You're gone."
Go to 3F
Once they reach 3F, the player hears Lord Slimy's footsteps coming up the stairs, and has to hide (they cannot go down the stairs, due to the footsteps coming from there). They hide in a locker, but are found by the janitor's grandson and the cleaning Grimer, Grimo. Grimo has to clean the lockers, so has the janitor's grandson help him, since Grimo cannot open the lockers. Grimo works late at night to clean more than his rival Grimer, Grimey.
Scary PA system
"At night, teachers' lounge, second floor. You hear broadcast from beyond grave. You're done for if you hear it. You can't leave. It's over for you."
Enter teachers' lounge on 2F
As the player walks towards the door, they hear a broadcast over the PA system, telling them that they cannot get out. When they approach the door, a man with his Slowpoke appears. The man tells the player that the speaker was broken so he had to fix it. However, his Slowpoke fell asleep in front of the door and he couldn't get out.
Stairs to nowhere
"Night, stairs, cursed. You go up and up, but still same place. You can't go down, either. Circle, circle. Can't go back."
Go to 2F
Once they reach 2F, the player feels a cold breeze. Even if they go up or down the stairs, they remain on 2F. If the player goes to the west side of the floor, they notice the bulletin board. One piece of paper has black stains on it, which indicates the lower left, upper left, lower right, then upper right corner. If the player steps on the sparkles on the floor in these corners of 2F in that order, they can use the stairs again. When proceeding to 1F, they are immediately attacked by a wild Gastly.
"It appears on the shelf, third floor. Keep reading, and scary thing happens. If you finish reading it, someone take you away."
Read book from shelf on 3F
The diary tells a story of someone who bought a seemingly haunted house. They would hear a constant scratching sound, which the player then hears. The owner would also see something moving in the house. After finishing the diary, the lights suddenly go out, and the player is attacked by a wild Drifloon. After completing the battle with Drifloon, the player reads the rest of the diary and discovers that it is in fact a horror fiction novel written in diary format.
"At night, classroom, second floor. You hear laughter. Ghost are taking a class, too. Everyone says so."
Enter classroom on 2F
When the player enters the classroom, there is a teacher and students in the room. Many of the students tell the player to join them and be their friend. If the player interacts with the teacher, she is revealed to actually be a Hypno creating an illusion, who then flees and disappears. The students, also part of Hypno's illusion, also vanish.
After the player solves all six of the offered mysteries, the Preschooler tells the player that she moved from a foreign country and didn't speak Alola's language very well. She was sad that her friends were scared by the mysteries, but now that the player has solved them her friends don't need to be scared, giving the player the Wise Glasses as thanks. Suddenly, the school's security officer calls out to them, questioning why they are at school so late at night. The player talks to the security officer, but the girl has vanished, and the officer tells the player that they were alone when he called out to them.
After this, the school will return to normal at night, playing the regular music and without the Preschooler and her Drifloon appearing.