The 19-year-old paraprofessional shot in the head outside a Brooklyn elementary school has died — with his accused killer now under arrest, police said Wednesday.

Javier Oates, also 19, was picked up in Brooklyn and charged with murder, criminal possession of a weapon and criminal use of firearm in the Tuesday shooting of Ethan Holder, a substitute at PS 203, according to police.

Holder was walking at Avenue M and Utica Avenue in Flatlands just before 3 p.m. when a gunman wearing a red ski mask shot him once in the forehead and once in the thigh.

He was rushed to Brookdale Hospital in critical condition and listed as brain dead Wednesday morning, according to sources. He was declared dead later Wednesday, cops said.

Oates allegedly confessed to cops after being taken into custody, police sources said, adding that the shooting may have been sparked by a personal beef between the two men.

A Flatlands neighbor called Holder “a very sweet kid.”

“He’s not the kind of kid who was into trouble. He don’t have gang friends or anything like that,” she said Wednesday.

“He’s usually just alone on his way to work. He just goes to college and work and you know you see him taking an Uber to go to class,” said the woman, who asked that her name not be used. 

“I don’t understand why anyone would shoot him,” she said. “This morning his mother called and I was just shocked. He’s a great kid and this is horrible news. I can’t believe it.”

She said Holder’s mother is from Grenada and his father, who she said works for the city Board of Education, is from Guyana. 

The attack took place not long after classes let out. Dismissal time at the school is around 2:30 p.m.

Footage from a nearby deli shows the suspect walking north on Utica Avenue — when he spots the victim heading west on Avenue M, the sources said. 

The suspect then walks east on Avenue M before stopping, turning around to face his target and firing at him, the sources said, citing the video.

The victim collapsed into the front door of the deli after he was shot. The gunman fled in a dark Lexus, according to cops and police sources.

The shooting happened within known gang territory, but the victim has no criminal history or gang ties, the sources said. 

In addition to a red ski mask, the gunman wore a red hoodie, red sweatpants and a red do-rag, police said.

“I am absolutely heartbroken and outraged that one of our dedicated employees was senselessly made victim to random gun violence in southern Brooklyn this afternoon,” wrote Department of Education Chancellor David Banks in a tweet. “Our educators, our students, and our families deserve better than living with the threat of gun violence.

“I am in contact with the school’s principal and will ensure that the students have the counseling services they need as they grapple with this tragedy and will be working with the NYPD to increase security around the school,” he added.

NYC Councilmember Mercedes Narcisse called Tuesday’s gun violence in her district “sad, very sad.

“But it’s sad because as a city council member, you don’t want these things happening in your community. You don’t want it to happen in the city. You don’t want it to happen at all,” she told The Post.

She worried that the elementary school-age children will have a hard time understanding Tuesday’s shooting.

“It’s not a junior high or high school where the kids can understand what’s going on,” Narcisse said.

“It’s just so sad. I mean the school is doing all the extracurricular activities trying to get the kids on the right track and this is it?” she added.