Ranpou

(from ebookjapan):

This absurd gag comedy depicts the crazy antics of the mysterious boy Ranpou and his unique friends. Ranpou is scolded by teacher Kakumaru for chatting with Chutaro the rat during class.

In fact, Ranpou, who was an honor student a month ago, has been taken inside a mysterious UFO and turned into a new Ranpou, different from the one before. Mutsumi, a female student who is interested in Ranpou, who may possibly be an alien, follows him to...!

Adapted into an anime in 1984. Link to the Boxset on Amazon.jp Here