Peggy and Bobby meet Katt and Michael at cotillion, who tell them that Michael is doing this because Michael wants her upcoming birthday party to be fancy and including all of this cotillion dress, poise, and behaviour. As soon as Katt says so, Peggy starts angling for an invitation to the party.
Yet they show up at the birthday going overboard with trying to dress and act cool and loud. Almost ever since they met Peggy and Bobby have known exactly what kind of party this was supposed to be, it is essentially how they met, it was previously established how Bobby was expected to dress and behave - using all the lessons he learned in cotillion - and Peggy should clearly have appeared in kind. Then the very next scene after hearing about the party, when they get the invite, Peggy talks about how they have to research how to dress and be cool. No confusion should have been possible.
Yet they show up at the birthday going overboard with trying to dress and act cool and loud. Almost ever since they met Peggy and Bobby have known exactly what kind of party this was supposed to be, it is essentially how they met, it was previously established how Bobby was expected to dress and behave - using all the lessons he learned in cotillion - and Peggy should clearly have appeared in kind. Then the very next scene after hearing about the party, when they get the invite, Peggy talks about how they have to research how to dress and be cool. No confusion should have been possible.