Before she was sentenced Thursday to a maximum of six years in a state juvenile detention center, the 15-year-old assailant of an autistic boy in Southern Maryland pleaded for leniency. The girl, who had used her cellphone to record the abuse, asked a St. Mary’s County judge if she could avoid jail and instead be placed in a community-based treatment program.

But Judge Michael Stamm sternly rejected the Chopticon High School student’s request, declaring that her actions and those of her accomplice — which included luring the autistic 16-year-old onto a frozen pond, where he fell in multiple times, and encouraging him to try to have sex with his family’s dog — were “horrific.” The 15-year-old, he said, posed a danger to the community.

In urging the judge to give the girl “intense long-term residential treatment,” St. Mary’s prosecutor John Pleisse said that authorities also have been investigating allegations that she and her friend bullied two other people — including a second disabled student at Chopticon.

“We’re still investigating [one youth], and we know about another disabled person that was taken advantage of,” Pleisse told Stamm, using the alleged victims’ first names.

The other girl involved in the case, Lauren A. Bush, 17, has been charged as an adult with first-degree assault, false imprisonment and child-pornography solicitation and, if convicted, faces up to 80 years in prison. Bush, who is accused of twice holding a knife to the boy’s throat, is undergoing several weeks of psychological examinations at a state juvenile detention center while she awaits a May hearing on whether her case should be transferred to a juvenile court.

This month, the 15-year-old pleaded guilty to second-degree assault and displaying an obscene photograph of a boy. Stamm said she will be held at a secure juvenile facility until age 21 at the most but could be released earlier if authorities believe that her rehabilitation is progressing quickly. (The Washington Post generally does not identify juveniles in criminal cases.)

In charging documents, St. Mary’s sheriff’s deputies say that the Chopticon girls repeatedly assaulted their schoolmate from December to March and that their acts were captured on the younger girl’s cellphone.

The 15-year-old, dressed in a bright-pink hoodie and shackled at her feet and hands, cried throughout Thursday’s hearing. The girl wrote an apology letter to the parents of the autistic boy, but when she rose to speak before the court, the girl did not say she was sorry and instead only asked to avoid a jail.

“I just feel a not-lockdown facility is going to be best for me,” the girl said.

“Of course you do,” Stamm interjected with obvious sarcasm.

The boy’s father, a federal government contracting analyst, took the stand and addressed the girl directly. “Your actions have betrayed the trust of someone who thought you were their friend,” he said. “I can say that [my son] almost never smiles, and I hope in time he will heal. I also hope that you will grow to understand that it is wrong to prey upon the weak in our community.”

In an earlier interview with The Washington Post, the boy said he wants to resume his friendship with the girls and does not believe that they did anything that warrants jail time. He believes that what the authorities called assaults were actually games, but his parents are furious and want lengthy sentences for both girls.

James Tanavage, the 15-year-old girl’s attorney, told the judge that his client takes responsibility for the attacks and that Bush was “more aggressive.” The 15-year-old, he said, is “in­cred­ibly remorseful about her involvement” and the fact that she became mixed up in a “perfect storm” with the “wrong person at the wrong time.”

Tanavage and the girl’s parents declined to comment.

After the sentencing, Sgt. Cara Grumbles, a spokeswoman for the St. Mary’s Sheriff’s Department, said the detective handling the autistic boy’s case interviewed the father of another disabled young man who had been bullied by the girls. The father, however, has declined to press charges on behalf of his son, who is now 22 and lives in another state.

His older sister told The Post in an interview that he’d lived near the 15-year-old and that both she and Bush harassed her family numerous times.

“They put my brother’s name on a gay Web site, and his address, and these two men showed up at our house,” said the sister, who requested anonymity in order to protect her brother. “We were having a lot of problems with [the younger girl] and Lauren coming over and destroying my dad’s property.”

Her younger brother, she said, attended Chopticon and has a physical condition that makes him “mentally slow and handicapped.”

“They would say he’s retarded,” she said. “I remember him coming home in tears because they’d tease him. They threatened to beat him up.”