The parents of a six-year-old from Colorado who was born a boy but identifies as a girl have filed a civil rights complaint against their child's former school.

Coy Mathis's parents say they suspected their child was transgender from an early age, but really began allowing her to live as a girl when she was three. Since they made the switch, they say their child has blossomed to become a happy little girl.

Coy's school allowed the first-grader to use the girls' washroom when she started kindergarten there. But last month, the school's principal told the family that would no longer be allowed. Instead of using either the girls' room or the boys' room, she would have to use the staff washroom instead.

Kathryn and Jeremy Mathis pulled their child out of school, and with the help of a transgender rights group have now launched a discrimination complaint.

The parents say they were stunned with the sudden reversal.

"We were really confused. We were hurt by the response. They had been so overwhelming accepting and then completely changed their mind after a year of allowing Coy to go to the school as a girl," they told CTV's Canada AM from Denver, Colorado.

The Mathises say they knew Coy was different from the time she was a toddler, because the tot, who's a triplet along with a brother and a sister, always tended to gravitate toward "girly" things, such as pink blankets and dolls.

"As early as 18 months, when she was able to start talking, we were able to see her likes and dislikes," her mother says. "At that point, we didn't think, ‘Oh, she's identifying as a girl.' We just thought we had a little boy who liked girls' things."

Coy's parents, who have five children all together, bought her the toys and clothes normally associated with boys, but she showed little interest. In fact, she refused to leave the house if she had to wear boy's clothes.

"It wasn't until she was quite a bit older, closer to four, that she was able to start telling us, ‘It's not that I like girls' things; it's that I am a girl,'" Kathryn recalls. "She was experiencing depression and anxiety trying to force herself as a boy when that just wasn't who she was."

When Coy asked to be taken to the doctor to be "fixed," her parents took her to a psychologist, who diagnosed her with "gender dysphoria," also known as "gender identity disorder."

So just before Coy started kindergarten, the Mathises agreed to let Coy live as a girl. They say they held a meeting with the school's principal, vice principal, kindergarten teacher, school counsellor and psychologist and they all agreed to follow Coy's wishes.

"After that, everything for Coy just blossomed," says her dad, Jeremy. "She became this spunky little girl and her grades started improving and she started talking about all the fun things she was doing in school instead of talking about how she didn't want to go to school."

In kindergarten, the washroom choice wasn't much of an issue since the children used a unisex bathroom. And when Coy entered first grade, the school allowed her to use the girls' bathroom for the fall semester. But her parents say something changed over the winter break.

"We got this phone call from the school that what was currently going on with Coy using the girls' room wasn't going to be allowed to go anymore," Jeremy says.

School district officials told the Mathises that Coy would have to either use the staff bathroom or the one in the nurse's office starting in January.

Coy's parents were furious and decided to pull her from the school.

Their daughter is now being home-schooled, along with her siblings. Her mother says while Coy is keeping up academically, "she's lacking the social component that she would be receiving if she were still in school."

With the help of a support group from other parents with transgender children, the Mathises decided to file a complaint filed with the Colorado Office of Civil Rights that alleges a violation of the state's anti-discrimination law.

They say that school districts in many states -- including Colorado -- have policies that allow transgender students to use the bathroom of the gender with which they identify. As well, Colorado has anti-discrimination laws that include protections for transgender people.

The Mathises admit the complaint has led to a lot of media attention for their family and for Coy. While the family has received criticism for subjecting their child to the media spotlight, the Mathises say it's the school district that's to blame.

"The main problem is that she's already been stigmatized by the school. She was forced out of the school and we're trying to hold the school accountable for what they've done," says Kathryn.

"And in order to do that we have to talk about it. The media is not something we ever wanted to do. We were put in this situation by the school not wanting to talk about it."

With files from The Associated Press