a prom is a formal dance held at the end of the years of high school, called junior prom and senior prom respectively. A few hours before arriving to prom, there is usually a "pre-prom" where a group of students and their parents, family members, and friends take pictures and get ready prior to when their form of transportation (sometimes a rented limousine, other times driven by a parent) arrives to bring them to prom.

A typical pre-prom gathering, with boys in tuxedos and girls in dresses with corsages.
Boys are usually dressed in tuxedos with bowties, sometimes with brightly colored cummerbunds or vests, though any sort of formal wear can be worn. Girls usually wear formal gowns or dresses adorned with a corsage given them by their date, and often dress to shock or be noticed in low cut dresses, shiny or brightly colored fabrics, unusual cuts, or formal pants outfits. Often times, boys and girls will dress according to the theme of the prom - i.e. pastel suits for a Miami Vice-themed prom.
Common prom activities include dining, dancing, and socializing. The prom night is laden with significance and anxiety to many students about being invited, finding a date, what to wear. On the other hand, some high school students feel the prom is the most romantic night of their lives. They may go in a group that includes a person they have known for years; other times, students just try to find a date that they like. Many find it to be just as fun to attend with friends, not worrying about the dating aspect of the prom. Because of the age of the attendees, parents, teachers, or others usually act as chaperones for the prom and after-prom activities. The use of chaperones is intended to prevent the occurrence of violence or alcohol/drug abuse.

That's what wiki says about the prom which everyone in the batch is looking forward to. Rawr.