Kodomo no Jikan - Season 1 Episode 1 A Friendly Step

It is the first day of teaching for Daisuke Aoki and his third grade class. During the course of the day, he learns about the reasons behind Mimi Usa's extended absence from school. Her absence is due to a long-term harassment by the previous teacher. Neither the other teachers nor her parents believed her, so Mimi's only choice was to quit school. This enraged Rin and Kuro, so they harassed that teacher to the point of causing the teacher to have a nervous breakdown during class and quit. Later, Rin tells Aoki that she hates adults because of that incident.