I said “The Prom” when I graduated in 1981. My 18 year old nephew and all the college students I work with refer to it as just plain “Prom”, which makes me want to give them a pinch.
We went “to prom.” I’m from Western Kansas, and graduated HS in 1986.
We also went to prom, to Homecoming, to Winter Formal, and to graduation. There’s nice parallelism in our set of Very Important Events.
(Southeast Wisconsin, class of '07)
Both are perfectly acceptable. SW Ohio mid-1980s.
Man, the disparity is amazing, isn’t it? I can’t quite get a handle on whether it’s generational or geographical, or both / neither.
AHA! This is extreeeeeemely helpful, Justin_Bailey, more than you can imagine! Because the book/story I’m editing is actually something of an [del]attempt to capitalize off of[/del] homage to the Twilight books. So knowing that these books use “the prom” gives me a good defense for wanting to make the change. 
Thanks to everyone who’s answered so far!
I’ll put in a small, purely anecdotal vote for generational; my mom went to the same high school as I did, and she always referred to “the prom”.