A group of parents and students composed of 51 families has filed a lawsuit against the U.S. Departments of Justice and Education for pressuring a school district in Illinois to allow a transgender "female" to use the girls' bathroom, saying the resulting compliance by the school district violated student privacy and safety.

The Alliance Defending Freedom (ADF) and the Thomas More Society filed the lawsuit on behalf of the families on Wednesday before the U.S. District Court in the Northern District of Illinois.

The lawsuit cited the education department for threatening Township High School District 211 with a loss of $6 million federal funding if it did not allow a transgender female to enter and use the girls' locker rooms, which the district consented to under an agreement with the department.

The new policy was based on the interpretation of the education department of Title IX, which provides that "[n]o person in the United States shall, on the basis of sex, be excluded from participation in, be denied the benefits of, or be subjected to discrimination under any education program or activity receiving Federal financial assistance."

According to the lawsuit, nothing in Title IX's text, structure, legislative history, or accompanying regulations addresses "gender identity," a phrase which also does not appear in the text of Title IX and its regulations.

It added that Title IX and its regulations use the term "sex" and not "gender identity" in describing the type of discrimination prohibited, adding that "sex" means male and female "under the traditional binary conception of sex consistent with one's birth or biological sex."

The transgender student in question attends the William Fremd High School, who requested the school to let him use the girls' locker rooms but was denied.

He filed a case with the Office of Civil Rights (OCR) and the district offered him to use private facilities which he refused.

The OCR sided with the student in July 2015 and told the district that he was entitled to use the girls' locker rooms.

It issued a Letter of Impending Enforcement Action to the district to remove millions of dollars in federal funding if it did not agree to allow the student to use the girls' facilities. The district relented.

"Protecting students from inappropriate exposure to the opposite sex is not only perfectly legal, it's a school district's duty," said ADF Senior Counsel Jeremy Tedesco in a statement. "Allowing boys into girls' locker rooms, a setting where girls are often partially or fully unclothed, is a blatant violation of student privacy.

He said the school district should rescind the policy and the court should order the education department to stop bullying school districts.

ADF legal counsel Matt Sharp said the education department exceeded what it is allowed to do, adding that at least five other federal and state courts have rejected the DOE's interpretation of Title IX.

The lawsuit is asking the court $1 million in nominal damages and to issue a preliminary and permanent injunction against the enforcement of the Locker Room Agreement and Restroom Policy.