Kyoukasho ni Nai

Tairaku is a high school teacher in a school full of bad apples, and as trying as his life may already be, a chance meeting with a young girl is about to throw it into chaos. His home life, his school life, and his tentative relationship with a fellow teacher whom he has a crush on, are destined to teeter on the brink, thanks to this one young student. (MangaUpdates)