I made up magic and taught a world-destroying witch?

Joseph transmigrated to a fantasy world where magic had declined, becoming a legendary archmage. The bad news was, he himself knew absolutely no magic and only wanted to collect some funds to escape as soon as possible. To avoid being exposed, he specifically recruited "problem students" expelled from other academies: a princess with zero sensory perception, a blacksmith's daughter with zero self-control. Couldn't learn? That was a matter of aptitude, not his problem. Then, he brought out the system from his previous life's games and started blindly making up spell slots, eight major schools, ring-level theories... and the result... the students actually mastered it. Looking at the student who conjured a stable fireball in his palm, Joseph was dumbfounded: "I made it up, how did they master it?!" This should have been good news, until he ran away with his funds. "Sir, if you don't become our teacher, this world has no reason to exist."