I’m a Teacher at a High Martial Academy

The male protagonist is reborn in a high-martial world. He assumes he’ll have a System to back him up and exceptional talent, only to discover he’s a good-for-nothing with nothing going for him. After graduating from a mid-tier academy, the guild sells him off to the front lines to fight the demon race. By sheer luck, he survives and scrapes by in demon territory for twenty years. When he finally returns, he’s a middle-aged man with mysterious, formidable strength. Now back in the human world, he hopes to use his abilities to live a carefree life, but the demon race has already infiltrated the schools…