yes she can wear anything that she likes but given the set up of the after party, I doubt she'll be able to enjoy it cuz of constantly fixing her dress
manga is about a girl and guy with the same name (super common name like it is used as an example in filling up forms in japan), both are plain and not good looking, both LOVE yakuza movies and one day, dressed up as one of them. there was a corruption within the school i.e. bullying and such...