Everyone Else is A Returnee

Briefly about Everyone Else is A Returnee manga:

They forgot to take him on a picnic in junior high school. It was forgotten on a camping trip in middle school.

Everyone Else is A Returnee manga.

He was forgotten on a trip in high school. He finally became a university student, and what? Now he was forgotten by all mankind? Yu Il Han, who is left alone on Earth after all of humanity is sent to other worlds to study. His story will begin when humanity returns and the Great Cataclysm happens!