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

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)