Sensei wa Erai!.

Nakajima, Chinen, and Arioka play three useless students at a public junior high school. At first, they find their homeroom teacher dull and bothersome, but an incident leads them to take a liking to him. Along with their former classmate (Yamada) who now attends a private school, the students unite to transform their teacher into a respectable guy. - (TokyoGraph)