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.