"It's wrong for schools to do that, it's wrong to discriminate," McMillen told The Associated Press.
McMillen's stand made her feel like an outcast in her Mississippi hometown, but the 18-year-old hasn't looked back. She has traveled the country making high-profile media appearances and visited the White House as an invited guest. On Sunday, she will be one of three grand marshals for New York City's annual Pride March.
"I never would have had these opportunities if I hadn't done what I did," she said.
One of things McMillen did was bring a lot of attention to her tiny Mississippi town.
Upfront about her sexual orientation, McMillen said she had approached officials at Itawamba Agricultural High School in Fulton, Miss., over the course of her senior year about bringing her girlfriend to the prom. Same-sex prom dates had been banned in the past, but she had hoped things would be different.
When she was refused, the American Civil Liberties Union got involved and McMillen filed suit.
McMillen said the suit made her a pariah in Fulton, lost her friends and made her finish her senior year at another school.
"It really is no one compared to how many friends I used to have," she said. "There's really nothing there for me."
But if Fulton has closed its doors to her, as she says, the larger world has opened up.
McMillen, who plans to go to college in Memphis, Tenn., to study psychology, has made appearances on TV programs including the "The Early Show," ''The Wanda Sykes Show" and "The Ellen DeGeneres Show." DeGeneres presented her with a $30,000 college scholarship from Tonic, a digital media company.
McMillen also attended the Gay and Lesbian Alliance Against Defamation media awards, sharing the room with the likes of "Glee" creator Ryan Murphy, actress Drew Barrymore and singer Adam Lambert. Earlier this week, she attended a White House reception with President Barack Obama and gay activists.
On Friday night, residents of Woodstock, N.Y., were throwing "All Love, All Woodstock," a fundraising event featuring singer Ronnie Spector and others, in McMillen's honor. Proceeds will go toward her college education and the ACLU.
Abbe Aronson, organizer of the Woodstock event, didn't know McMillen before the controversy. She said she was compelled to host the event after reading about the teen's determination to stand up for herself.
"I was reading about this young woman who had basically been really stomped on by both her peers and the school and refused to give up," Aronson said. "I was just struck by how graceful she was and how gracious she was given how badly she had been treated."
The example McMillen set is why she was asked to be grand marshal in New York City's Pride March, said Arthur Finn, co-chair of Heritage of Pride, which organizes the march and other related events.
"She's really a courageous young woman who's been willing to stand up for herself and all of us," he said.
The other marshals are Lt. Dan Choi, an openly gay National Guardsman, and Judy Shepard, mother of hate crime victim Matthew Shepard. The three represent the struggles that gays and lesbians still face, Finn said.
"A lot of people sometimes think that the fight's over," he said, "It's not over, and these three exemplify it."
McMillen, who has never been to New York City's pride events, said she was "happy to be part of such a great celebration."
And while she's been enjoying her recent experiences, she said the most important thing she wanted people to take away from her story was the importance of speaking up and being heard.
"I don't care if people know who I am," she said, as long as they know "there was this girl that did stand up."