Sou Itte, Kimi wa Warau

Synopsis

Having forgotten to hand in his science paper, Sera goes looking for his teacher in the staff room. Unfortunately for him, his teacher isn't alone when he finds him. He's with Sera's classmate Sasaoka, and to Sera's shock, the two of them appear to be lovers. Having secretly witnessed something he shouldn't have, the incident leaves Sera with confused feelings... (Source: MangaUpdates)