After you stop in at Los Platos, you head north into the big city of Mesagoza, where you will have your first day of school.
There is a Sugar-N-Ice shop west of the southern entrance to Mesagoza.
There is a Seabreeze Café west of the southern entrance to Mesagoza. If you check behind the building, you find a Potion and Super Potion.
North of Seabreeze Café is an area with many restaurants, as well as a vending machine where you can get Fresh Water, Soda Pop, and Lemonade.
Throughout Mesagoza, you will find Deli Cioso, Sure Cans, and Artisan Bakery, where you can buy ingredients that you can use for making sandwiches after you unlock picnics; Delibird Presents, where you can buy various items, such as Poké Balls and phone cases; Barato's restaurant; Seabreeze Café; Every Wich Way, where you can learn new sandwich recipes after you win Gym Badges; Kanto Crepes; Chic Kebab; Go-For-Broke Grill; Smoochurro.
In the central plaza of Mesagoza, there is a hair salon; a Bagin's shop where you can buy backpacks; Picnic-Knacks, where you can buy picnic goods such as tablecloths and cups; Chansey Supply, where you can buy battle items; Rough & Tough, where you can buy gloves; Capbourg, where you can buy hats; Spec Shack, where you can buy glasses; Sock Quarter, where you can buy socks; Zapaldea Footwear, where you can buy shoes. You can press left on the directional pad to change clothes any time.
If you go east from the western Pokémon Center, you find a ladder. Climb up it, then go south onto the rooftop to find TM072 ELectro Ball.
There is an Ether east of the western Pokémon Center of Mesagoza, but you have to go around to the east side of the buildings to find it.
On the stairs south of the central plaza, there is a woman who can give you Rotom Phone cases if you have save data from other Pokémon games, such as Pokémon Legends: Arceus, Pokémon: Let's Go, Pokémon Sword and Shield, and Pokémon Brilliant Diamond and Shining Pearl.
There is a ladder onto a roof northeast of the central plaza. Climb it to find a Zinc and TM010 Ice Fang.
In a courtyard east of the central plaza, you will find TM081 Grass Knot behind a tree.
In the park in southeastern Mesagoza, there is an item west of the pond.
In the plaza west of the eastern Pokémon Center, there is a Soothe Bell.
In the northeast corner of the northeastern grassy area of the city, there is a Full Heal.
Meet Team Star
When you approach the area just south of the long stairway up to the Academy, you'll find someone being accosted by Team Star. The Team Star Grunt has a level 8 Shroodle. You get 416 Pokédollars for winning.
Afterward, you receive the Tera Orb, and you will battle another Team Star Grunt, who has a level 9 Yungoos. You can Terastallize your Pokémon by pressing R before you select a move in battle. If you use moves that are the same type as the Tera Type of your Pokémon, the moves will be much more powerful. Some Pokémon's Tera Types are very different from their usual type, so be sure to check. If a move matches your Pokémon's Tera type, the type symbol of the move will have a spinning line around it. You get 468 Pokédollars for winning.
Recharge the Tera Orb
Be sure to heal up at a Pokémon Center to recharge your Tera Orb. You won't be able to Terastallize again in battle until you do so.
Go to the Academy
When you're ready, go to the north part of Mesagoza and go up the stairs. Before you go to the front door, you can find a Heal Ball to the east of the Academy building. Then go to the exclamation mark on the map and go through the door to enter the Academy.