I Make Up Secret Arts and Teach Them to a World-Destroying Demoness?

Lin En transmigrated to another world where magic was in decline, becoming the dean of a failing arcane academy in the North. The bad news was that he couldn't use magic at all. He just wanted to swindle some royal funding, then run off with the money once the academy went under. To keep anyone from discovering the truth, he only accepted “problem students” expelled from the top academies: A down-on-her-luck noble girl with zero elemental affinity, a blacksmith's daughter with no control whatsoever, a frail abandoned infant with a weak bloodline, a shadow-branch scion cast out by her family... Can't learn? That's a matter of talent, nothing to do with me as dean. Then he brought out the rules from the games he'd played in his previous life and made things up with his eyes closed: A nine-circle magic system, a meditation method for the mind sea, the principle of elemental resonance, an explosive combat style, a dual-affinity cultivation method... And then... the students actually mastered them. Watching his students conjure stable fireballs in their palms and casually blast away half the training ground, Lin En gaped in disbelief: “I made all that up! How did you actually learn it?!” It should have been a good thing, until the day he finished collecting the funds and prepared to run away: His students blocked the academy gates, with the entire Kingdom Mage Corps, elven envoys, and dwarven master smiths standing behind them. They all bowed in unison: “We beg you to stay, Dean. The Holy War of the Continent is about to begin. Please take command of the allied armies as their Supreme Mage!” Lin En was completely dumbfounded. He really was just a funding scammer. How had he become the continent's universally acclaimed progenitor of all magic?