Sally really has a whirlwind night at Miss Porter’s. The sort of bullying and partying Sally goes through was typical of the era, and still is today, really.
Sally wears a prim blue plaid dress with a wide white collar and bow to meet and greet the faculty and students at Miss Porter’s. She’s also wearing blue plaid when she catches Don in the act; this outfit (and trip to boarding school) is a direct response to that incident. Her blue colors are a distinct opposite from the headmaster, who wears orange, and the other girls, who wear yellows, reds, and pinks. Later during the ride home, Sally is still in blue, this time a navy overcoat.