Here is more detail about the alleged assault, as described in the the complaint:
[I]n November 2017, a boy known to the school administration to identify as “gender fluid” (“the Assailant”) was permitted—pursuant to and as a direct result of the Policy—to enter the girls’ room while [Victim] was there. While the two young children were in the girls’ restroom alone together, the Assailant confronted [Victim], pushed her against a wall, and forcibly touched her genitals despite her protests, causing her both pain and fear. This sexual assault (“the Assault”), which was a foreseeable result of the Policy and would not have happened but for the Policy, discriminated against [Victim] based on her sex and created a hostile and intimidating environment in which [Victim] must fear repeated incidents of sexual harassment or assault in the future.According to the Washington Post, "City Schools of Decatur officials have contested the version of events laid out in the complaint," including by "contradicting claims that the classmate the girl identified is gender fluid." It also noted that "a social service agency investigation determined the girl's allegations were 'unfounded.'"
This is an investigation we will watch closely and with concern. The Department of Education repealed the previous administration's guidance that requires schools to accommodate transgender students according to their gender identities in bathrooms and locker rooms, but it has not prohibited schools from doing so. If the agency finds that the school has violated Title IX, it could signal the agency's position that inclusive bathroom policies violate Title IX. It would also create a conflict between the agency and judicial interpretations of Title IX which have in recent years consistently supported transgender students rights in bathrooms, and which have rejected arguments on behalf of cisgender girls that the inclusion of transgender girls somehow violates their rights.