Hidetoshi Nakata

Hidetoshi Nakata, simply known as Teacher, is Kiyomaro's math teacher and homeroom teacher at Mochinoki Middle School.
In the first episode of the anime, Mr. Nakaka appeared to have a more serious and intimidating appearance and appears younger, given he had brown hair at the time. Sooner in the anime, Mr. Nakata was redesigned to look more like his manga counterpart, appearing slightly chubbier, older (with brownish-gray hair), and appears to be more comical.

(Source: Gash Bell Wiki)