prom proposal
n. An elaborate or unusual request to attend a high school prom with someone.
Examples
2006
Pity the boys in today's celebrity-driven, over-the-top entertainment culture, where asking a girl to the prom has turned into performance art.

Prom proposals, as these humbling exercises are now called, have been more elaborate than ever this spring, according to Promspot.com's associate editor, Kate Wood. Promspot solicited examples this year and received hundreds of responses from teenagers all over the country, "even North Dakota," says Wood. "This is not just an East Coast/West Coast thing."

Clearly, though, it is a big thing. A chat with her girlfriends, a phone call or a quick conversation by the lockers between classes won't do anymore. That's so 2005. In 2006, the request has to be painted on a giant sign parked in front of her house or accompanied by 50 red candles, hundreds of Hershey Kisses and an original poem. Why? For the same reason guys go to prom: because girls want it that way.
—Laura Sessions Stepp, “To Pop the Question, Kids Are Thinking Big; 'Wanna Go to Prom?' Is Now an Epic Request,” The Washington Post, May 19, 2006
2005
Devising a plan to ask someone to the big event has become a big event itself.

Tales are rampant on Mountain Pointe and Desert Vista high school campuses about guys, for example, who sprinkle rose petals along the walkway to a girl's home before presenting her with a dozen red roses and a prom proposal. Then there's the boy who used paper cups to spell out, "Will you go to prom with me?" in the chain link fence along Loop 101.
—Susanne Tso, “Popping the prom question takes creativity,” The Arizona Republic, April 16, 2005
2001 (earliest)
The two had a contest to see who could be more creative in their prom proposal. A few weeks ago, Leah asked David by sending him on a scavenger hunt.
—Mhari Doyle, “Roadside prom proposal,” The Columbian, June 01, 2001