Kiama Kiichi

Mitsuya-san is one of the few long-lived demi-humans in Japan. She is not very familiar with modern life, so in order to relearn, she became a high school student at the age of (approximately) 200 years old!

An alternate world (isekai) school gag manga. The manga follows Yamada, a company employee who is around 30, and a young demon king.