On Thursday, a day after the Itawamba County school board did just that, the 18-year-old lesbian high school senior reluctantly returned to campus to some unfriendly looks, she said.
"Somebody said, 'Thanks for ruining my senior year."' McMillen said.
The districtit wouldn't host the April 2 prom, saying allowing the girl-only couple to attend would violate their policy, reports CBS News corresopndent Mark Strassmann.
The decision came after the American Civil Liberties Union demanded that officials change a policy banning same-sex prom dates because it violated students' rights. The ACLU said the district's rule prohibiting McMillen from wearing a tuxedo violated her free expression rights.
Tune into CBS' "Early Show" Friday morning to hear McMillen discuss the case.
The ACLU filed a lawsuit in U.S. District Court in Oxford to force the school district to sponsor the prom and allow McMillen to bring whom she chooses and wear what she wants.
District officials didn't returned numerous calls left by The Associated Press seeking comment on Thursday.
McMillen said she never expected the district to respond the way it did.
"A lot of people said that was going to happen, but I said, they had already spent too much money on the prom" to cancel it, she said. "I'm just trying to get done what I originally wanted done. Now, we're having to fight just to have a prom."
McMillen said she didn't want to go back to Itawamba County Agricultural High School in Fulton the morning after the decision, but her father told her she needed to face her classmates, teachers and school officials.
"My daddy told me that I needed to show them that I'm still proud of who I am," McMillen told The Associated Press in a telephone interview. "The fact that this will help people later on, that's what's helping me to go on."
A school board statement said it wouldn't host the event in Fulton, "due to the distractions to the educational process caused by recent events" but never mentioned McMillen or her girlfriend, who also is a student at the school.
Same-sex prom dates and cross-dressing are new issues for many high schools around the country, said Daryl Presgraves, a spokesman for GLSEN: Gay, Lesbian and Straight Education Network, a Washington-based advocacy group.
"A lot of schools actually react rather than do the research and find out what the rights of these students are," said Presgraves, who was preparing to facilitate a discussion about anti-gay bullying at a National Association of Secondary School Principals meeting.
U.S. Rep. Jared Polis, D-Colo., said a bill he's introduced in Congress would protect students such as McMillen. Polis said the measure would make it illegal to discriminate against gay and lesbian school students. He said his bill is modeled after similar laws in at least 10 states.
"This situation with the prom is a perfect example of why we need to protect students from discrimination. In this case it's a prom. It other cases, it's getting beaten up or killed," Polis said.
Polis said he was "dismayed" by the school board's action.
"They ruined the prom, not only for this young woman, but for all of the straight kids at the school," he said.
The school district had said it hoped a privately sponsored prom could be held. McMillen said if that happens, she's sure she'll be excluded.
"It's a small town in Mississippi, and it's run by an older generation with money. Most of them are more conservative and they don't agree with it," she said.
Fulton Mayor Paul Walker said he supports the school district's decision and knew of no private efforts to host the prom.
"I think the community as a whole is probably in support of the school district," Walker said of the town of about 4,000.
Southside Baptist Church Pastor Bobby Crenshaw said he's seen the South portrayed as "backwards" on Web sites discussing the issue, "but a lot more people here have biblically based values."
Itawamba County is a rural area of about 23,000 people in north Mississippi near the Alabama state line. It's near Pontotoc County, Miss., where more than a decade ago school officials were sued in federal court over their practice of student-led intercom prayer and Bible classes.
A couple of students had different reactions to the decision.
Anna Watson, a 17-year-old junior at the high school, was looking forward to the prom, especially since the town's only hotspot is the bowling alley, she said.
"I am a little bummed out about it. I guess it's a decision that had to be made. Either way someone was going to get disappointed - either Constance was or we were," Watson said. "I don't agree with homosexuality, but I can't change what another person thinks or does."
McKenzie Chaney, 16, said she wasn't planning to attend the prom, but "it's kind of ridiculous that they can't let her wear the tuxedo and it all be over with."
Presgraves said his organization hears about school districts that prohibit same-sex prom dates and gay-straight alliance clubs at schools. He said those kind of policies are detrimental to gay students.
"It sends a message that these students shouldn't be treated the same," Presgraves said.