(CNN) -- Philip Chism, a 15-year-old Massachusetts boy accused of raping and killing his math teacher in a high school bathroom, was arraigned Wednesday on attempted murder charges stemming from a separate attack on a female youth services clinician.
Prosecutors laid out a narrative of what they said was "the defendant's intent to murder" a female employee last month at the Dorchester Department of Youth Services facility where Chism is being held pending trial, the Suffolk County District Attorney's Office said.
A judge set bail at $250,000 and ordered Chism to not have contact with witnesses, including the victim. He is to appear in court September 19 and an automatic "not a youthful offender" plea was entered on his behalf.
Assistant District Attorney Mark Zanini said that on June 2, Chism was in the facility's main room, which is bordered by a low wall. A staff member was seated at a station behind the wall, able to look down into the room. Behind the staffer was a hallway used by staff.
Zanini said Chism had refused to attend classes while in custody. Instead, Chism was instructed to sit and study at a table in the open room. Despite a rule that he sit at the table closest to his room, Chism allegedly sat at a table that provided him a view of the hallway behind the staffer, the statement said.
At some point, the 29-year-old female victim, whom Chism had known for several months, walked down the hallway and entered a bathroom inside a staff locker room, according to the statement.
Chism was "confident the other staffer was distracted" and kicked off his sandals, which would have made noise on the floor, before moving toward the hallway in a crouch that kept him out of the staffer's view, Zanini said at the arraignment.
Chism had a pencil in his hand and opened the door to the locker room, according to the statement.
When the female clinician came out of the bathroom, Chism stared at her from about a foot away. He placed both his hands around her neck and began to choke her while pushing her back against the cinder block wall in the bathroom, according to the statement.
The victim was unable to scream because Chism's hands tightly gripped her throat, Zanini said.
When she managed to get his right hand away from her neck, Chism punched her in the face, head and jaw, prosecutors said. She screamed as other staffers rushed to help her and restrain him, Zanini told the court.
The victim suffered bruises to her face, jaw and head, along with a scratch on her back consistent with a tear in her shirt that appeared to be from the pencil Chism carried, according to the statement.
Chism was charged with attempted murder by strangulation, assault with intent to murder, kidnapping and two counts of assault and battery with a dangerous weapon: a pencil and the cinder block wall.
Chism's attorney, Denise Regan, declined to comment.
Chism is also accused of killing his algebra teacher, Colleen Ritzer. Chism, 14 at the time, used a box cutter he had taken to school to kill Ritzer, 24, in October 2013 in a girls' bathroom at Danvers High School, police and prosecutors said.
Chism pleaded not guilty to aggravated rape, robbery and murder charges in January. He is being tried as an adult.
After Chism was reported missing by his mother on the evening of October 22, police had his cellular phone company "ping" the location of his phone. The phone was found to be near the Hollywood Hits Theater, where investigators learned the teen had purchased a movie ticket and then left.
A court affidavit said that when Chism was spotted by a police officer the next day, he was carrying a knife; a search of his backpack turned up a bloodstained box cutter. Asked where the blood came from, Chism allegedly responded: "The girl."
Police said he was carrying credit cards and a driver's license belonging to Ritzer, as well as a pair of women's underwear.
Police discovered Ritzer's body in a wooded area near the school, covered with leaves and debris in an apparent attempt to conceal it, court papers said. Her throat was slit.
Chism's case for the alleged assault on the youth services worker is scheduled for a pretrial conference on September 19, but his presence for that event has been waived, according to the statement.