Sore wa Marude Amagasa no You ni

Cheering on her friend's crush, using abusive language towards the homeroom teacher. Yayoi's fun school life changes greatly after one incident. The one who saved Yayoi, who was deeply hurt, was her homeroom teacher, Tsuzuki-sensei. The forbidden love between a teacher and a lonely high school girl!

(Source: Kodansha, translated)