Saori blew a hole in Hieronymus the same way she blew a hole in Sensei
If you went up to this point in the story, the team Sensei summoned with his card are not the students that are helping him right now. Rather the students summoned by him is implied to be "his" students from another place (in game you're force to use your own students for this boss fight instead of using story characters, making this boss fight huge gatekeep for low level accounts) hence why this Saori is no longer wearing her mask and is perfectly okay with her face reveal.
