Brennan had said the girl was a threat to the community based on the assault allegation involving her mother in November, according to court documents.
The girl also allegedly stole a cellphone from a fellow student at Birmingham Groves High School in Beverly Hills, northwest of Detroit.
The girl is being called "Grace" to protect her identity. She was placed on probation in April and, among other requirements, was to complete her schoolwork.
Grace, who has ADHD and receives special education services, struggled with the transition to online learning and fell behind.
When Brennan sentenced the girl to detention on May 14, she told her that she was sending her to Children's Village to get treatment and services.
"She was not detained because she didn't turn her homework in," Brennan said Monday. "She was detained because she was a threat to her mother."
"My role is to make decisions that are in this young lady's best interest, period," Brennan said. "I took an oath that I would not be swayed by public clamour or fear of criticism."
Birmingham Public Schools said last week that it had no role in the case, but that no student should be punished for missed online work during the coronavirus pandemic.
The district switched to virtual instruction following a state-mandated shutdown of schools in March.
The ProPublica story was also published by the Detroit Free Press and Bridge, another news organisation.
Grace is black, and news of her case prompted protests over how black children are treated in the criminal justice system.
"People need to learn how to work with our kids," said Monique Campbell, who is African American and participated in a protest Monday outside the court building. "There needs to be equitable treatment of all kids, and we need to realize there isn't."
Vivian Anderson, founder of EveryBlackGirl Inc, also attended the protest and said black children are being criminalised for "behaviour that's accepted in other communities as a way of life - this is their ageing process."
- AP, additional reporting, NZ Herald