Baka Demo Kuzu Demo Ii janai

Chikako, who tends to fall foolishly head-over-heels in love, starts attending English conversation classes to (without him knowing) go along with her boyfriend on his overseas work assignment.
Yoshinaga, her teacher, is a playboy who gets with all the girls! What a scumbag!
Hoping to go abroad, Chikako tries hard at English class, and she is actually improving a great deal, but it turns out that her boyfriend wasn't even taking their relationship that seriously.
Chikako, heartbroken in despair, sleeps with Yoshinaga...!?

An opposites-attract love story between a girl who is too honest and a boy who can't be honest at all.

(Source: Shogakukan, translated)

Note: The tankoubon volume also includes a three-chapter story Otona Shikaku (大人失格) and the one-shot Baka demo Kuzu demo Ii Janai: Kore Made de Ichiban Sukina Hito (バカでもクズでもいいじゃない～これまでで一番すきな人～)