I wonder...
If they use the treasures of the previous demon kings to fund the school then will they not run out of money eventually?
After all the fact that they need to use the previous demon kings' treasures means that the current demon king does not earn / gather enough treasure to pay the costs themselves.
Unless, of course, the part about the treasure of previous demon kings being used as funds is just an excuse to make the MC go and explore the dungeons to train and level up.