I'M THAT MONSTER GIRL YOU ONCE HELPED OUT: A MIDDLE-AGED TEACHER FINDS UNEXPECTED POPULARITY IN ANOTHER WORLD

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)