The Institut Villa Pierrefeu -which claims to be the only traditional finishing school in Europe that is still operating-used to teach debutantes proper deportment.
Not only would Queen Charlotte and her ladies-in-waiting be wearing this style, but all of the debutantes presented to her in court would be expected to wear this type of garment.
The term " debutante" refers to any wealthy (that is, rich) young woman who is first appearing in society, who first starts to go to fancy parties, to what we call " balls" (balls).
When we meet Allie Hamilton, she is 17 years old and would be considered a sub debutante, which was a term used for a girl in the years just proceeding her debut into upper-class society.