Two suburban Chicago students filed a lawsuit in U.S. District Court claiming their high school violated one of the students' civil rights by not letting her wear an anti-gay T-shirt last year.

Heidi Zamecnik, 17, of Naperville, and Alexander Nuxoll, 14, of Bolingbrook — students at Neuqua Valley High School in Naperville — filed the lawsuit seeking court permission to openly express their opinions on homosexuality during the National Day of Silence, scheduled for April 18.

On the Day of Silence, students can refrain from speaking as an effort to protest discrimination against homosexuals.

In response to a Day of Silence event at the school in April 2006, Zamecnik wore a shirt that read "MY DAY OF SILENCE, STRAIGHT ALLIANCE" on the front and "BE HAPPY, NOT GAY" on the back, according to the suit filed Wednesday.

According to the suit, one school administrator ordered Zamecnik to remove the T-shirt and another official ordered her to cross out "NOT GAY" with a marker.

The suit alleges Zamecnik suffered unlawful discrimination and humiliation because school officials didn't agree with her viewpoint. Nuxoll did not attend the school at the time of the incident.

Calls by The Associated Press to the Indian Prairie School District and Neuqua Valley High School were not immediately returned Thursday morning.

The Arizona-based Alliance Defense Fund, a conservative Christian litigation group, is representing Zamecnik.

"At its core, this is about students' fundamental right to express their beliefs, no matter how controversial or whether they are in the minority or not," said Nate Kellum, an Alliance Defense Fund attorney in Memphis, Tenn., who is representing the two students. "This is a fundamental First Amendment issue."

Gary McCaleb, senior counsel for the group, said the organization has filed at least eight similar lawsuits nationwide. McCaleb said the Alliance Defense Fund is trying to "enable Christian students to express a contrasting viewpoint on homosexuality."

McCaleb said Zamecnik and her parents discussed the incident with school officials to work out an agreement allowing the teen to wear a similar T-shirt during next month's Day of Silence event. The suit said school officials declined the request.