American college degree courses generally last for four years, named in order of completion, freshman, sophomore, junior and senior.
A 'prom' (short for 'promenade') is a ball or dance held by a particular class at a school or college.
The 'Yale Junior Prom' was a prestigious annual social event held (usually in January or February) from 1870 until the 1970s. It took place over several days and, as well as a formal dance, included events such as parties, dinners, concerts and sports matches.
Dorothy Parker is reputed to have said, 'If all the girls who attended the Yale prom were laid end to end, I wouldn't be a bit surprised'.