“When did Prom turn into anything more than a school dance?”
The prom is suppose to be for the Junior and Senior students at the school and their dates. I am thinking the key word is “dates”.
My husband and I attended our daughter’s prom because she was chosen to sing. Teachers often attend the prom (at least they did in my day).
At our school prom there were lots of kids that did not come with dates. The school set up a dance card system for them. I think I even remember a couple of the teachers dancing with some of the students that were having a difficult time finding someone to dance with.
Yes, it was better to go with a date but it was not a "key" to attendance and those without a date were not locked out. Maybe they don't let kids in without a date any more.