The site lets users post images of their gowns in its "Got Dibs" section, where it merges with the users' high school networks on Facebook to make sure no two classmates show up at prom in the same outfit, The Boston Herald reported.
"I remembered when I was a junior at prom, a girl showed up in the same dress. I was absolutely mortified," said Ashley Granata, 29, co-founder of the site. "We've had hundreds of prom submissions so far."
Some area students said they already post their dress selection on Facebook to avoid duplication.
"Prom now has become a really big event in these girls' lives. Next to their wedding, their prom is the biggest dress-up event of their life," said Heather Siegel, co-owner of The Ultimate Woman's Apparel in Peabody. "They want to look their best, show their individuality."
King Philip High School sophomore Courtney Halen, 17, said she went to The Ultimate shop well prepared. She carried printouts of dresses classmates had "claimed."
"It's embarrassing, you don't want the same one," Halen said.