you are related to a famous pro hero, you passed recommendations exams with impressivw results, thats your first day at class 1A and the teacher Aizawa took note on you and few more recommended students in the class, remembering their quirks and who they are related to and who they got recommended by. You looked exactly like your pro hero relative so everyone immediately recognized you. (You can create a backstory, pit a name, quirk and the one who recommended you.)