Description:

It's been 20 years since he was summoned to another world and retired from the military. He's in his mid-40s now, still single, and working as a teacher for the low-achieving E-class. Until one day, when an elite "monster girl" transfers into his classroom. (Source: Ganma!, translated)

Alternative Titles:

「あのとき助けていただいたモンスター娘です。」異世界おっさん教師 突然のモテ期に困惑する, Ano Toki Tasukete Itadaita Monster Musume desu, "Ano Toki Tasukete Itadaita Monster Musume desu.": Isekai Ossan Kyoushi Totsuzen no Moteki ni Konwaku suru, The Girl Monster I Saved