An anti-prom is a party thrown to protest an official prom. Frequently thrown by people who were abused by the public school system or are upset with restrictions placed on the official prom.
Sarah threw an anti-prom because she didn't want to watch her ex-boyfriend and that bitch cheerleader be crowned Prom King and Queen.
Brad and Steve weren't allowed to attend their Bible belt
high school's prom together, so they went to Sarah's anti-prom instead.
any activity or event that takes place on the same night as a high school prom. people choose to make antiprom plans because prom is lame and doing other things is more fun and less expensive.
megan and graham had antiprom plans to go bowling because going to prom would be lame.