The one event that exceeds Homecoming in importance is Prom, a formal dance held for upperclass students. Traditionally, this was as an opportunity to learn social graces and was seen as an entry into adulthood. In modern times it is simply a social event, albeit a culturally important one. For some students, Prom is the highlight of the entire high school experience.
Prom is only open to students in the last two years of high school, though younger students can attend with an older date. Getting a date is important - unlike the Homecoming dance, it is relatively rare to attend Prom alone.
As mentioned, Prom is highly formal. Students will obtain formal clothing specifically for the event, purchase gifts for each other, hire limousines and go to expensive restaurants before the dance itself. It's common for friends to attend Prom in groups along with their dates.
Every Prom has a romantic theme selected by a committee in advance. Both of the upper class grades raise money for Prom, and as such it tends to feature far more elaborate decorations than less formal events like Homecoming.
Presentation aside, Prom is quite similar to other dances. The main distinguishing trait is that most of the students in attendance are about to graduate. As with yearbooks and graduation parties, Prom is a last chance to spend time with old friends.
An increasingly common event is After Prom, an all-night lock-in party that follows the dance. After Prom was created as an alternative to student parties which tend to include heavy alcohol consumption.