Carla Hale, 57, said she was told she was being let go because her relationship is against teachings of the church.
She plans to file a complaint this week with the city of Columbus, which prohibits firings based on sexual orientation, her attorney said Monday. She already filed a grievance that is now in the hands of a union representing teachers in the Roman Catholic Diocese of Columbus.
Some current and former students have rallied behind the physical education teacher, staging a protest outside the diocese headquarters and starting an online petition that has collected about 100,000 supporters.
Hale said she was fired during Holy Week in March after an anonymous letter sent to school administrators drew attention to the obituary published in The Columbus Dispatch.
A copy of the letter provided by her attorney was signed "a concerned parent."
"My daughter came home and told me that one of the gym teacher's mother had died," the letter said. "She asked me to pray for her. When we looked in the obituaries, I was shocked by what I saw. It had her teacher's name and that of her 'spouse' listed. It was two females!"
Hale, who is Methodist, was informed about two weeks after her mother's death that the school was investigating, but she never had a chance to discuss it with school leaders, said attorney Thomas Tootle.
Hale, who had spent 19 years teaching at Bishop Watterson High School, said the decision to acknowledge her partner was not immoral.
"It's kind of baffling that someone would take an obituary and use it, to me, in such a mean-spirited manner," Hale said at a news conference last week.
The Diocese of Columbus would not comment directly about the firing, but it said school employees can't go against teachings of the church.
"All Catholic school personnel at the outset of their employment agree that they will abide by the rules, regulations and policies of the Catholic Diocese, including respecting the moral values advanced by the teachings of Christ," the diocese said in a statement.
Hale's attorney said he will file a complaint Tuesday with a Columbus community relations board, arguing that the firing violates the city ordinance on employers discriminating based on sexual orientation. Another option is a wrongful termination lawsuit, Tootle said.
He said some courts have allowed religious groups exemptions to similar discrimination laws, but he thinks the case is similar to one in Cincinnati where a teacher challenged her firing by the archdiocese over her use of artificial insemination to become pregnant. A federal judge has allowed that lawsuit to continue.