A gang leader who was convicted thanks to testimony from rapper Tekashi 6ix9ine was given a 17-year sentence Monday — despite his former high school teacher’s suggestion that he work as a mentor for local kids instead.

Judge Paul Engelmayer imposed the sentence on Aljermiah “Nuke” Mack after noting he had done “nothing to accept responsibility” for his leadership role in the violent Nine Trey Gangsta Bloods set.

Had Mack — who was convicted of drug dealing and racketeering — pleaded guilty to his crimes, Engelmayer would have reduced his sentence by several years, he said in court.

“Remember this moment,” the judge told Mack when lecturing him to take responsibility for his crimes.

Before the sentence came down, Pedro Sierra, a Bushwick High School teacher and dean, sent a letter of support backing Mack, describing the gang member as “well respected in the community.”

Sierra wrote that he met Mack while teaching at the school in 2000, and found that he treated “me and the other adults with the utmost respect.”

“I know deep down in my soul, if giving the opportunity Mr. Mack will spend his time giving back to our community,” Sierra wrote.

He added that Mack could “start” at the school’s campus if he avoids prison, and help impoverished students there.

“Many of the students are also facing many of the same traps Mr. Mack has faced, however, Mr. Mack will be able to help them avoid it,” Sierra wrote.

“In this day and age many of the students will listen to someone who is well respected in the community as Mr. Mack,” he added.

Mack was convicted of racketeering conspiracy and conspiracy to distribute heroin and fentanyl after rainbow-haired rapper Tekashi 6ix9ine testified against the gang members he previously associated with.

Mack, a high-ranking member of the Nine Trey outfit, allegedly helped bring a prolific drug dealer into the gang and knowingly sold fentanyl mixed with heroin.

He pleaded not guilty in the case, but Tekashi fingered him as a leader of the violent Brooklyn Bloods set.

Mack, who faced life in prison, had also pleaded with the judge to go easy on him — pointing to his troubled upbringing in Brooklyn and the fact that he has a young son.

“My life is in the palm of your hands, judge,” he said.

“The community I grew up in is socially isolated. I only had two options: death or a jail sentence,” he added.