Lion to Hanayome

Yua is a high school girl in a secret relationship with her teacher. But one day her grandmother, her last surviving family member, passes away and leaves her all alone in the world. And then, her beloved Sensei, supporting her again as he always has, asks her to marry him. She's convinced that she's about to begin a happy newlywed life, but it turns out Sensei has a huge secret...?! At school and at home, she's gotten tangled up in some complicated afterschool marriage blues!!!

(Source: Akita Publishing)

Included one-shot:
Volume 2: Sukidarake no Kimi