Halfway through Lauren Bradley’s senior prom, the high school principal asked her date to leave. “He had pink hair,” Bradley recalled. “A pink Mohawk that she did not approve of.”

Now Bradley is one of the librarians who helps plan the New York Public Library’s annual Anti-Prom. It’s a party for students who may not feel comfortable at their own senior dance. About 400 students go each year.

The event, which is held at the library’s Stephen A. Schwarzman Building on June 8, is free to anyone between 12 and 18-years-old. There’s music and dancing — but also a few twists on the old theme, like a batch of puppies that made an appearance last year, and the crate of free books that arrives at the end of the evening.

The idea to throw a prom unlike a regular prom grew out of a conversation between a group of librarians in 2004. They were talking about their terrible prom nights, when one suggested the New York Public Library throw its own. “No,” another replied. “We should throw an Anti-Prom!”

They billed it as “prom for the rest of us,” a dance without the usual rigmarole of corsages and hairdressers. There’s a fashion show featuring teen designers from the High School of Fashion Industries, and elaborate costumes on the dance floor. Last year’s theme was “gods and goddesses.” One teen arrived in a wheelchair with cardboard waves spinning along with his wheels. Two attendants accompanied him in togas and laurels. “He was Poseidon,” librarian Caitlyn Colman-McGraw said. “It was so great.”

Another year, a girl in a Little Red Riding Hood cape walked down the runway with a basket. She made a few rounds, then opened it to reveal the head of the wolf who tried to eat her. That’s Bradley’s personal favorite.

Colman-McGraw plans to attend this year’s “Under the Sea” prom in scuba gear. It’s a classic prom committee theme, she said, but she’s hoping to feature some lesser-known fish.