By Fiona Ortiz

CHICAGO (Reuters) - The U.S. government on Monday found that a Chicago suburban high school district discriminated against a transgender student and gave the school a month to provide full access to girls' locker rooms or lose federal funding.

The student, who has not been named, and the American Civil Liberties Union, which brought a complaint on her behalf, applauded the findings, while the school district called them "serious overreach."

After an investigation stemming from a 2013 complaint by the ACLU, and months of negotiations, the U.S. Department of Education's Office for Civil Rights found Township High School District 211 was violating federal non-discrimination rules.

The district says transgender students may use their gender-identified locker room if they change and shower privately. The government said a separate changing place was discriminatory because it subjected the student to stigma and different treatment.

The case is seen as clarifying federal rules on locker-room access at a time of expanding awareness of transgender issues.

In mid-October the school district, with five high schools and two alternative high schools west of Chicago, defied the government, continuing to deny full locker-room access for the transgendered student.

Assistant U.S. Secretary for Civil Rights Catherine Lhamon said the high school was disobeying the law. "All students deserve the opportunity to participate equally in school programs and activities – this is a basic civil right," she said in a statement.

"This decision makes me extremely happy – because of what it means for me, personally, and for countless others," the student said in a statement released by the ACLU. "The district's policy stigmatized me, often making me feel like I was not a 'normal person.'"

The school district said the issue was critical for schools nationwide. Superintendent Daniel Cates said in a statement that "what we offer is reasonable and honors every student's dignity."

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Last year, the district received $6 million in federal money contingent on compliance with non-discrimination rules.

The student in the case has identified as female for years; the school lists her as a female student and she plays on the girls' sports teams and uses girls' restrooms.

The school district has provided the student with a separate changing facility outside the locker room and installed privacy curtains on stalls in one locker room out of the three that she uses for physical education, swimming and athletics programs, according to the federal government's findings.