The moment he left the portal and arrived at his destination, Mew got to work. And with Mew, he meant his cute little Pokemon.
Having gathered a lot of psychic, ghost and flying type pokemon, Mew sent them out in teams to search for his former students. He had handed out their pictures and made sure they wouldn't let themselves be seen by the people of this world.
When each pokemon was on their way, Mew entered the slightly dilapitated school building. Frowning a bit at the state, he and a few others (pokemon and human from the garden) went to work to renovate it a bit.
Things like renewing rotting wood, cleaning up dust, etc. The usual stuff.
When evening came, a lot of the pokemon he had sent out had returned as well, each having done their task of handing out the invitation.
The date had been set for the day after tomorrow, giving Mew enough time to decorate and gather all the necessities to host a barbecue party that afternoon. He didn't have to worry about the weather either, as a simple weather changing move/abillity would fix it up nicely.
Like that, the day was soon upon him, as the children (now adults) were slowly gathering.
From the more withdrawn of the lot, to the arrogant looking Karma. By the time that the set time was upon them, nearly everyone had arrived. All but Nagisa. Even Karasuma and Bitch-sensei were present.
[Hmmm... I suppose Gengar got lost?]
Mew muttered, as he clearly remembered he had sent Gengar to pick Nagisa up. Ah well, they should arrive soon.
And he was right.
A shadow alongside the forest swirled unnaturally, as the figure of a slightly pale Nagisa and a grinning Gengar appeared.
"Remind me not to go with him again *Urgh*"
Nagisa grumbled, holding a hand to his mouth to avoid emptying his breakfast.
[Does it matter? It should be rather pleasant if you get used to it]
Mew answered in turn, not understanding that traveling through the shadows might be more than a little bad for a regular human.
"I rather not. It is good to see you again, Mew."
Nagisa said, as he gathered himself back together and joined the rest of the class.
[Now then. Since everyone is here, I guess the star of the show should arrive as well.]
"I am already here though"
Karma playfully replied, grinning widely.
[Haha, very funny. Have fun catching up with each other.]
Mew retorted, as out of nowhere, the banana yellow octopus called Koro-sensei appeared.
He still had a silly grin on his face, alongside with the well-known pink color.
"""Koro-sensei!"""
The class exclaimed in joy, startling the daydreaming takoyaki.
"Nyehehe?!"
Twitching a little in surprise, a wide toothed smile appeared on his face at seeing his class again.
"It seems you have done rather well in the time I was 'dead'"
Koro-sensei replied, getting an angry but happy yell in retort, as green bullets were shot towards him.
Easily dodging the anti-hero-sensei bullets, Koro-sensei smirked in delight as he appeared in multiple places to enjoy the wide variety of food, as well as entertain his former sudents.
It was a fine day indeed for a happy reunion.