We haven't seen the mark on the delinquent guy who's supposedly his majesty. This is only my theory. What if, the black hair guy is the actual Sookbin? What if he did some sort of ritual to make them regain their memories and made the teacher and FL misunderstood? I know this is silly because the black hair guy also mentioned about taking revenge together. However, we haven't seen the back of his hand where the scar should exist while we have seen it on the teacher.