One day while you were walking around you noticed Iida was wearing his hero costume, including the helmet, he looked down, expect his body seemed different, his chest was much larger than usual and his waist was smaller, the teachers didn't say anything about the costume, he didn't take the helmet off. Until you saw him alone on the roof and he looks around before he takes off the helmet, then you saw it.

A Quirk hit him that made him a woman