The country singer's eyes popped open in surprise when she caught sight of her mother, Andrea, in the crowd of gown-and-tuxedoed guests dressed up as an over-the-top gym teacher-chaperone in a dowdy black dress, white socks, sneakers, and false teeth.
Props? She had hung a whistle around her neck and clutched a clipboard marked with the stern warning: "No Kissing On The Dance Floor!"
"Taylor told me I would have to wear one of those big dresses and I just couldn't do it," Andrea told PEOPLE. "But I wanted to go along with the theme. I came up with this instead. She didn't even recognize me when she came in!"
Swift, 18, attended the Hillcrest High School prom in Tuscaloosa, Ala. on April 19 as part of MTV's Once Upon a Prom program (airing June 12th) but missed her own Hendersonville High gala on May 3.
She specifically asked for a prom theme for the fete, which was to celebrate her first No. 1 hit, "Our Song," and the multi-platinum status of her self-titled debut album.
But according to Andrea, the evening was all about the dress – a baby blue ballerina-styled confection of satin and tulle.
"She is wearing the dress she wore in the video for 'Our Song'," says her mother, "She loves that dress and always said it would make the perfect prom dress, so this was her chance."
Swift accessorized with sling-back silver shoes, a rhinestone tiara, a wrist corsage, and a school ring on a gold chain around her neck.
Is there a secret steady for the girl who says she hasn't kissed a boy in months?
"No!" she said, with a laugh.
"My mom gave [the ring] to me to wear. I guess it used to mean you were going steady? I think it might be my dad's school ring."