made up magic and taught a world-destroying witch?

Joseph didn't ask to be reborn as a legendary archmage in a world where magic had all but faded into myth. Unfortunately, he knows absolutely nothing about the arcane arts. His only goal is to quietly amass enough coin to flee before anyone discovers his secret. To keep up appearances, he handpicks a class of so-called "problem students"—rejects from other academies who are deemed too hopeless to teach. Among them are a princess who cannot sense magic at all and a blacksmith's daughter whose impulses are beyond control. If they fail to learn, well, that is simply a matter of personal aptitude. It is hardly his concern. To fill the void of genuine knowledge, Joseph dredges up the mechanics of a game system from his past life. He begins inventing spells, constructing elaborate theories of the eight schools of magic, and fabricating ring-level classifications out of thin air. Yet, to his absolute shock, his students actually master the nonsense he spouts. When one of them conjures a stable sphere of fire in her palm, Joseph can only stare in disbelief—he made it all up, so how in the world did they learn it? This revelation should have been a triumph, but Joseph has no intention of sticking around. With his hard-earned funds in hand, he makes a break for it. He does not get far. His students find him, and their message is chillingly clear: if he refuses to continue as their teacher, then this world has no reason to exist at all.