Fliers were posted all around the school, announcing:
WINTER HOLIDAY PARTY
IN THE GREAT HALL
13 DECEMBER, 7 PM
LIVE MUSIC
INTOXICATING BEVERAGES
BRING A DISH* TO SHARE!
*A food dish, not slang for an attractive female
The Sorting Hat was sure this would sound immensely appealing to everyone. After all, this was Hogwarts. Everyone loved a good party.
What the Hat hadn't written (or rather, commanded Dictaquills to write) on the fliers was that the 'live music' was a karaoke machine, the 'intoxicating beverages' were cans of Red Bull, and the promise of a potluck was just a ploy to lend verisimilitude to the idea there was a real party going on.
Inside the Great Hall, the hapless partygoers would find not balloons and confetti, but tables reconfigured into a factory sweatshop setup. There were sewing machines; there were markers and paper and paint; there was, incredibly, glass-blowing equipment; and there were house-elf foremen armed with the cattle prods the Hat had formerly employed back during its
family values kick.
The Hat had a plan. The plan was to get rich quick by manufacturing cheap unlicensed Chudley Cannons merchandise and sell it at little stands in Diagon Alley and on platform 9 3/4 King's Cross Station. Unpaid slave labor would totally come in handy, and the Hat would need most of the house elves to actually hawk the merchandise. This was where the Hogwarts students would come in very handy!
Needless to say, once a partygoer entered the Hall, they would not be allowed to leave, much less send owls or any other form of warning to people who hadn't yet arrived. They would be at the Hat's mercy! Mwahahaha!
(Actually they would be at the mercy of armed house elves acting on the Hat's instructions, since the Hat itself couldn't be bothered, and would rather hang around the Sorting Room extorting bribes from applicants.)
(( Happy HHversary! You may NPC house elves instructing characters on how to make Chudley Cannons t-shirts, posters, stained glass window-ornaments, whatever. ))