In order to create you own school, you need to get to combat trial level 50. You do this by doing PVP and gaining a total of 250 points. 1 for participating, 1 for defeating your opponent at least two times, and 1 for defeat your enemy three times for the victory. This takes a considerable amount of time even if you win all your matches. I suggest you get lucky and find someone to boost with or just grind it out. Do at least 10 levels a day and you'll get there in no time.