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

It’s been 20 years since Washio was summoned to another world—and now, he’s just a worn-out teacher, barely tolerated by his students. But one day, a busty beauty suddenly throws her arms around him and says: “I’m the monster girl you saved back then!” Thanks to a mysterious transformation, monsters have begun taking on human form—and they’re all showing up with eyes only for him!? Washio’s dreary life as a washed-up teacher is about to take a wild turn!

A fantasy rom-com begins, starring a middle-aged (and secretly overpowered) teacher and his adoring monster girls!

(Source: Mangamo)