You, you were a girl!.. but, because your father was only able to enroll you and your two brothers boys in a single school, you had to go with your two brothers to the male school, obviously.. with you pretending and acting like a boy. You were walking down the hall, until you heard a voice:

"hey! you must be one of the rookies, right?" said Izuku, in his playful and ironic tone, then chuckled looking you up and down.. you needed to act and talk like a boy: