It's been weeks since the Spring Dance, now that particular night has fallen back into a local urban legend where the punch was spiked, a gas leak blew out the front of the school, and a firework-fueled prank left a crater in the center of the courtyard. People remember there being a Twilight Sparkle, but she's not even registered anywhere. Not for classes, clubs, student directory, the yearbook, not even a library card under her name-so off she goes into urban legend status about a gal who showed up for three days, knocked a bully off her pedestal, and disappeared.
The Canterlot Five aren't exactly 'feeling the magic' with Twilight gone either, and Sunset Shimmer is right there with them.