A teenager charged with the fatal stabbing of a beloved Brooklyn social justice activist was a “respectful” pupil who recently graduated from high school, according to a former teacher.

Brooklyn High School For Leadership and Community Service teacher Paul Joseph told The Post on Friday that he taught Brian Dowling, 18, in his senior English documentary film class — and personally congratulated him at his graduation.

He also described Dowling as “respectful” and “like any high school student.”.

“He was a respectful young man. So I don’t know what happened,” Joseph said, recalling that he shook Dowling’s hand at graduation..

“He just graduated — literally walked across the stage,” he said.

Dowling was arrested Thursday, when cops found a knife believed to be the alleged murder weapon and a sweatshirt matching the one the attacker wore at the teen’s home, police and sources said.

Photos of Dowling show tears running down his face – and cuffs around his wrists and ankles – as he’s escorted from the 81st Precinct stationhouse in Bedford–Stuyvesant, where he was charged with murder and criminal possession of a weapon.

The accused killer was wearing the same white T-shirt and striped shorts he had on when cops busted him at his home on Lafayette Avenue near Malcolm X Boulevard — down the block from where Ryan Carson, 32, was fatally stabbed early Monday as his girlfriend watched in horror.

“Assailant has been caught,” Carson’s father, Ken Carson, wrote on Facebook late Thursday morning.

“More details will follow,” he later added.

“I’m on the road… escorting Ryan Home to Massachusetts right now. I wasn’t leaving NYC without him.”

Dowling made no statement to cops after his arrest, law-enforcement sources said.

Sources previously identified a “mentally disturbed” teen as a main person of interest in Carson’s brutal slaying. The activist was attacked near a B38 bus stop just before 4 a.m. Monday.

Carson – a longtime campaign organizer for the New York Public Interest Research Group – and his girlfriend were heading home from a wedding on Long Island when the senseless attack occurred, police said.

The assailant seemed to target Carson at random, forcing the community advocate to stand between his girlfriend and the menacing stranger, troubling surveillance footage obtained by The Post showed.

The unhinged man can be heard yelling “I’ll kill you!” before the two scuffle, with Carson eventually tumbling over the bus stop bench while the attacker pulls out a knife and stabs him three times, according to cops and video of the carnage.

Carson lay crumpled on the ground when the assailant turned on his girlfriend and spat on her.

An unknown woman — who police said knows the suspect — then appears in the corner of the frame and seems to beg the knife-wielding man to not “hurt him” and repeats what sounds like “Brian.”

Carson died from a stab wound that pierced his heart, officials said. He would have turned 32 on Friday, records show.

By Wednesday, police said they had identified the suspect but were waiting for “probable cause to make this arrest,” NYPD Chief of Detectives Joseph Kenny said.

Police said the suspect may have been fighting with a woman – identified by friends as his twin sister, Brianna – shortly before he was seen on camera kicking over and damaging mopeds and scooters that were parked on the sidewalk.

He then turned on Carson, seething, “What the f–k are you looking at?”

Sources said the teen works at a high school in Clinton Hill and was known to frequent the area of Commodore Barry Park in Fort Greene.

It is unclear if he had any juvenile record, which would be sealed.

But Dowling was issued three summonses in 2022 — including two for disorderly conduct — and was a robbery victim in 2021, sources said.

He was also cited in a report related to a July incident in which his aunt claimed he smashed objects in his girlfriend’s apartment after a fight — and described him in a 911 call as mentally disturbed, sources said.

Eric Riddick, 58 — calling himself a best friend of the suspect’s dad, Brian Dowling, Sr., and someone who considered the Dowlings his “second family” – told The Post he always knew the teen as a good kid, with no previous red flags that would’ve explained the deadly attack he’s now accused of.

“I mean there were really no issues — where you can go back and check if there were any kind of problems with him in the past,” Riddick said.

Still, Riddick said Dowling’s “mental state” should be looked at.

Riddick said he’d spoken with the teen just “a couple of weeks ago on this block and everything looked fine,” adding that Dowling, who was “heavy into sports,” had just graduated from high school and most recently worked in construction, with a previous job at a restaurant.

He said he was on the phone with Dowling’s dad earlier Thursday and showed up at the home because “he was so distraught, he couldn’t speak to me.” 

Two young women, who described themselves as best friends of Brian’s twin sister Brianna – but also friendly with Brian – also went to the Dowlings’ house. When asked about the suspect’s mental health, one replied: “I do know there have been issues.”

She refused to give further details before getting consent from the family.

A few hours after Dowling’s dramatic arrest, one neighbor, who asked not to be identified, told The Post “it was complete shock to hear about what [Dowling] did.”

“I mean what was he even doing roaming the street at that time of day, with a knife?”

The individual said he lived in the neighborhood all his life, and used to know Dowling’s family “better.” He remembered the accused teen as “a good kid.”

“Today with the young kids you know, they get all their stuff from the internet, all these bad influences and they don’t wanna hear what we got to say,” the person lamented.

“I mean…one day you could be dead close to the person and they’re thinking murderous things or got a murderous mind. I mean that’s major.”

Other neighbors agreed that “of course” Dowling’s arrest for the slaying is “shocking.”

“Imagine if it was someone from your block,” said Riccardo Sanchez, 51.

Alyssa Vladimir, 39, added that after reading about Carson’s activism work, “it seemed like if he would’ve lived, he would’ve advocated for [Dowling] not to be incarcerated like that…so that I think is the most tragic part about the whole thing.”

“We don’t hang out outside at 4 a.m. … but we heard about it through the news and there was all this commotion outside,” she said. “I feel really bad. He made one really bad decision and that’s going to affect him for the rest of his life.”