A Bit of Background

In the United States and Canada a prom, short for promenade, is a semi-formal (black tie) dance held at the end of an academic year.... The name is derived from the late nineteenth century practice of a promenade ball. The end of year tradition stemmed from the graduation ball tradition. - Wikipedia

Proms first began in the elite colleges of the Northeast, taking their cue from the debutante balls of the rich and well bred. Middle class parents admired the poise and composure of debutantes and their escorts and began to institute formal dances as a means of instilling social skills and etiquette in their children. The dances were strictly chaperoned and were often restricted to only the senior class.

The formal dance we now call prom was first christened “the prom” in the 1890s. The word prom is a shortened form of promenade, meaning a march of the guests at the beginning of a ball or other formal event. - http://www.prettyforprom.com/prom-history.html