Cotillion ball

In American usage, a cotillion is a formal ball and social gathering, often the venue for presenting débutantes during the débutante season – usually May through December. Cotillions are also used as classes to teach social etiquette, respect, and common morals for the younger ages with the possibility of leading up to a débutante ball. For different places, there are different cotillions. Today, most cotillions are for middle schoolers as a chance to teach manners and etiquette, and also are a time to socialize with friends at after parties. The after parties at cotillion usually feature food, drinks, and music.

...
Wikipedia