Remember, kids today have cell phones, social networks and all that jazz, so it wouldn't be a stretch for kids to filming the incidents at prom as they're happening.
Birnbaum has implied the new film will be, at least partially, a found-footage horror movie. Now, before you get out your anti-found footage pitch forks, this shouldn't be that surprising. King's original novel is interspersed with various "official" documents recounting what happened on Carrie's deadly prom night (a writing technique called epistolary, which is basically the literary equivalent of found footage), so if Aguirre-Sacasa's script is indeed going back to the source material, the film focusing on "interviews with the survivors of the prom incident" makes perfect sense. It also doesn't necessarily mean Carrie will be filled with first-person POV found footage as we traditionally know – though the person who posted the recap did confirm to us that Birnbaum specifically used the words "found footage" – but is simply using recorded interviews as a framing device much like the book used newspaper clippings, which actually means Peirce's film will have more in common with the 2002 made-for-TV Carrie movie starring Angela Bettis than De Palma's film.Source: Movies.com