After that arrest, investigators seized three computers and two phones, and used text messages — which they said included inappropriate conversations about nude beaches and threesomes — as well as videos and photographs to identify six additional victims, ranging in age from 13 to 19.

Image Sean Shaynak

Joseph Mancino, a Brooklyn prosecutor, told Justice Martin P. Murphy on Tuesday that Mr. Shaynak had “looked to groom these students” with the text messages, trips, alcohol and cigarettes. He gave one student perfect scores on classroom quizzes, though she routinely left questions blank, Mr. Mancino said.

Mr. Mancino said that Mr. Shaynak had a four-month sexual relationship with one of the six students, whom he once took to a sex club in a different state, where he had sex with other patrons. He also took her on what the prosecutor called a “terror ride” from Queens to Brooklyn, when the student became so frightened that she ran from the car and hid from Mr. Shaynak in some nearby bushes while he was “screaming and banging on the car.” Mr. Shaynak also threatened to tell the girl’s parents about their relationship, Mr. Mancino said.

Another student accompanied Mr. Shaynak to a nude beach in New Jersey, after Mr. Shaynak told her they were going to pick up school supplies, law enforcement officials said. They exchanged 10,000 text messages, in which he repeatedly mentioned the nude beach. She also visited his home, where he gave her so much hard liquor that she passed out, officials said. Mr. Shaynak encouraged that student to have sex with another female student, Mr. Mancino said.

The sexual encounters occurred after the two students reached the legal age of consent, 17 in New York, but in one instance, he forced one of them to have sex against her will, prosecutors said.

While law enforcement officials recited the details of the new charges in court, Mr. Shaynak stood silently. His lawyer, Kimberly Summers, told the judge that her client denied any wrongdoing and that he had no previous criminal history. Mr. Shaynak had worked on special projects with certain students, she said, adding that one of those students “was routinely contacting him.”

Justice Murphy set bail for Mr. Shaynak at $1 million bond or $600,000 cash, which he had not posted Tuesday afternoon.