"It was an attack - a vicious attack - and in my opinion unjustifiable," said Mayor John Linder.
Confusion reigned at Chester District Court as Judge Dawn L. Vann first arraigned the four teens in a closed courtroom, then arraigned them again in open court after reporters challenged the closed arraignment.
The judge, who is not a lawyer, had no comment.
"They violated their rights, everyone's rights," Fern Davis, mother of two of the suspects, said of the closed-door hearing. "It is unheard of."
District Attorney Jack Whelan arrived in Chester to help clear up the confusion.
"No matter where the [hearing] is done, it should be done in open court," said Whelan.
The video, which shows the attackers hooting and dancing triumphantly, appears to have been posted as a sort of visual validation. A parent brought the video to the attention of Chester police Thursday after she saw it on the social-networking site.
The assault, as shown on the video, began with one girl walloping the head of the victim as she sat, defenseless, on the low front step of her Chester house.
Then a pack of half-dozen girls swarmed in and whaled mercilessly on the victim. They rushed inside her Morton Avenue home and threw her to the sofa.
The victim did not report the crime that night, but did seek medical attention for cuts and bruises, police said.
Police said they were also looking into another possible unrelated assault on the victim earlier in the month.
Whelan said he did not hesitate to try the teens as adults.
"The message is clear, if you are going to act violently, if your behavior is equivalent to animalistic behavior, then we will treat you accordingly, we will treat you as adults in the criminal-justice system," said Whelan.
Contact Mari A. Schaefer at 610-892-9149, firstname.lastname@example.org or @MariSchaefer on Twitter.