Look, if Superman can call himself just a journalist, and Spiderman can call himself just friendly neighbourhood, rather than friendly often-international-sometimes-extragalactic, let him have this.
i mean there is no down side for him taking part in the duel, he never wanted to join the school in the first place, so leaving is not a down side and if he wins what better way to get info on the hero then to have the heroes descendant as a slave?