Sore wa Marude Amagasa no You ni

From an award winning web-novel of the same name. Cheering on her friend's crush, using abusive language towards the homeroom teacher. Suzumina's fun school life changes greatly after one incident. The one who saved Suzumina, who was deeply hurt was her homeroom teacher, Miyazuki-sensei. The forbidden love between a Japanese teacher and a lonely high school girl!

(Source: Chibi Manga)