Dozens of families are taking legal action in a bid to stop a suburban Chicago school district from allowing a transgender student to use a girls' locker room.

Attorneys for conservative groups Alliance Defending Freedom and Thomas More Society filed the lawsuit Wednesday in Chicago federal court. They represent 51 families with links to the Palatine-based Township High School District 211.

The district last year agreed to let a student who was born male but identifies as female access the girls' locker room. Federal officials had found the district violated the student's rights. Under the agreement, the student can use a separate changing area in the girls' locker room.

Wednesday's suit says the policy "tramples" on students' privacy rights.

The American Civil Liberties Union calls the lawsuit a "sad development."