"Ano Toki Tasukete Itadaita Monster Musume desu." Isekai Ossan Kyoushi Totsuzen no Moteki ni Konwaku suru

「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 / The Monster Girl I Saved. / I'm That Monster Girl You Once Helped Out: A Middle-Aged Teacher Finds Unexpected Popularity in Another World

Some 20 years have passed since the protagonist was summoned to another world and retired from the army. He is still single in his 40s and lives as a low-ranking professor, until one day, an elite "monster girl" transfers into his class.