Lawrence Joynes shortly after his arrest on pornography charges in February 2013. (Baltimore County Police)

Former music teacher Lawrence Joynes pleaded guilty Monday to sexually abusing 15 minors, culminating a sweeping investigation into his conduct with students he taught in Montgomery County elementary and middle schools.

Joynes, who taught for 27 years at 11 Montgomery schools, officially accepted responsibility Monday morning for abusing students in kindergarten through second grade while he worked at New Hampshire Estates Elementary School in Silver Spring, Md.

A few hours later, he did the same in a second case involving a onetime student at Eastern Middle School in Silver Spring.

Joynes, 56, who has been held since his arrest in 2013, answered judges’ questions at the proceedings. At a morning hearing, he bowed his head, his forehead in his hands, as a prosecutor detailed the accusations involving the elementary school children.

Much of the abuse at New Hampshire Estates took place during school hours inside his music classroom, where he put girls in sexual poses and made video recordings of them — movies he edited with sexually explicit captions, according to police.

Joynes allegedly formed a group of students called the “lunch bunch,” earning their trust and recording some of them as they licked candy canes. He was accused in court papers of getting the name of one of victims tattooed on his shoulder.

The 15 counts to which Joynes pleaded guilty Monday related to 15 victims.

The case shocked parents, and court filings detailed one of the most alarming recent cases of child abuse in the Washington region involving a teacher. The abuse dates to the 1990s, when he allegedly preyed on a girl younger than 15 at Eastern Middle School.

At Monday’s afternoon hearing in the Eastern Middle school case, Joynes pleaded guilty to a charge of child abuse that prosecutors said encompassed a course of conduct with abuses alleged over a period of almost two years.

Police and prosecutors say Joynes gained that child’s trust, communicated with her in coded language and went on to increase his sexual contact with her. Police said that contact built from kisses and fondling to sex acts in his car and ultimately intercourse, which occurred weekly, including behind a stage in a music room.

“This man is a predator; he’s been a predator for decades,” State’s Attorney John McCarthy said after the hearings, noting that Joynes victimized “at least 15 children that we’re aware of. I think that’s a disgrace.”

Joynes is scheduled for sentencing in late August for the New Hampshire Estates case and shortly afterward for the Eastern Middle School case. He faces the possibility of spending the rest of his life in prison.

Mary Siegfried, Joynes’s attorney, said Monday that Joynes did not physically molest the 14 students at New Hampshire Estates. She said in court that he pleaded guilty under “exploitation” provisions of the law.

Montgomery detectives became aware of Joynes when he surfaced in early 2013 during a federal investigation of a child pornography ring. The federal investigation led authorities to Joynes’s home in Dundalk, Md.

According to court documents, school system personnel records showed two complaints about Joynes’s alleged inappropriate conduct at New Hampshire Estates. Joynes’s principal in 2011 placed him under restrictions, saying that he could not touch or be alone with students and that he had to leave his classroom door open during instruction, stay off the playground during recess and refrain from sitting with students in the cafeteria.

Police allege that he was abusing students until shortly before his arrest in February 2013.

Janis Sartucci, an advocate who attended Monday’s hearings and is a leader in the county’s Parents’ Coalition, called the case “unconscionable.”

“What we heard,” she said, “was that he was in his classroom exploiting these children and [Montgomery County Public Schools] did not stop him or remove him.”

Schools spokesman Dana Tofig said in a statement that the school system “has made, and will continue to make, significant improvements to our practices, policies and protocols related to preventing, recognizing, and reporting allegations of child abuse and neglect.”