Being strict with oneself and one's student with 心を鬼にする

Idiom Explanation: Taking a hardhearted stance with 心を鬼にする

Conan’s classmates aren’t fond of their new teacher strict attitude. Ms. Kobayashi explains how at her last school, she couldn’t bring herself to be strict and it allowed her students to get out of control that students were injured in a fight.

小林(こばやし):: 「だから、この学校(がっこう)では二度(にど)とあんな事(こと)は繰(く)り返(かえ)すまいと…」; "Therefore, to keep from repeating that kind of situation at this school..."; 「心(こころ)を鬼(おに)にして自分(じぶん)にも先生(せんせい)にも厳(きび)しくしてきたの…　」; "I hardened my heart and became strict, with myself and with my students..."