The incident was captured on video outside Primos Elementary School Sunday afternoon. The video was released by police Wednesday.
Police say the mom is 45-year-old Michele Davenport.
In the video, the girls seem reluctant. But Davenport has them square off and start swinging, and she can be heard yelling instructions from the sidelines.
After a while, the girls get tired of the struggle and actually try to shake hands. But mom signals round two.
Police say Davenport claimed her daughter had been bullied, and she came to help her settle a score.
"Mom still does not believe this is a big deal," Upper Darby Police Superintendent Michael Chitwood said. "She can't understand why she was arrested."
Davenport's daughter was also arrested.
The victim was treated at the hospital for a concussion and other injuries.
Police say they have zero tolerance for this kind of thing, especially since the recent death of 12-year-old Bailey O'Neill in nearby Darby Township.
He fell into a coma after a schoolyard fight and died two weeks ago.
Both Bailey and the victim in this case were classmates at Darby Township Elementary School. One was an altar server at his funeral.
Police say it's tough to send home the anti-bullying message when parents act like this.
"Mom's teaching the kids the wrong thing," Chitwood said. "Mom acted like a moron."
The daughter was released to the custody of her father. Davenport spent the night in jail and is now free on 10 percent of $35,000 bail.