New York’s justice system took one on the chin.

A 15-year-old high schooler used his experience as a boxer to save his teen gal pal from a Brooklyn subway mugging by a pair of beer-chugging migrants Tuesday — only to see one of the goons promptly set free by a judge after the attack.

Prosecutors had tried to hold Nelson Sanchez Sierra, 29, on $10,000 bail, as his charges in the robbery were eligible for bail under New York’s reform laws — but Criminal Court Judge Nancy Carrington declined the request and sprung him.

“That’s not fair,” the stunned teen victim, who asked to be identified only as Oliver, told The Post on Thursday. “How can he be released after robbing and assaulting a minor?”

The sophomore at Progress High School in Williamsburg, said he had been shopping with his 15-year-old friend friend and was about to go for pizza when he spotted the suspects stumbling into the Morgan Avenue station lugging a case of beer.

“They sat one seat away from us,” he said. “After that one of them got up, went to the side and looked at what I had. They started speaking to each other and the other guy sat closer to me. That’s when I told the girl to go away because I think they’re gonna do something.”

With his friend trying to get away, Oliver also tried to move down the platform— but he was soon stopped by Sanchez Sierra and his alleged accomplice Alexis Moreira, 34.

One of the alleged crooks threw a beer bottle at a train at the station to menace the youngster — and then started assaulting him. But the teen — who has both high school wrestling and boxing experience — tried to fight back, until he saw what he thought was a weapon.

“Once I got up they pushed me into the seat and then I got up again. I got pushed to the wall. I was trying to resist but then I saw him flash something. I thought it was a gun,” Oliver said. “It was scary once I saw a black thing.”

Fortunately, the attackers weren’t armed, he said.

But the trouble wasn’t over as the goons allegedly turned their attention to the teen’s female pal — and he had to act quickly to protect her.

“After everything happened, I was walking toward the girl . . . then they started walking towards her,” Oliver said. “I got in front of the guy . . . I told her to walk faster. They left us alone after that.”

A brief video shot by his friend shows the two migrants trying to grab his bags — but the teen said he was eventually able to snatch it back from the alleged crooks.

The suspects were busted when cops showed up, Oliver said.

Both were charged with robbery, assault, grand larceny and harassment, and arraigned Wednesday, at which time Sanchez Sierra, a Honduras native, was released without bail.

Moreira, who is listed as being in the US illegally, has yet to see a judge.

Brooklyn prosecutors had asked that Sanchez Sierra be held on $10,000 cash bail or a $20,000 bond.

Judge Carrington — who ordered Sanchez Sierra be freed under supervised release — was appointed to the bench by Mayor Eric Adams in September.

Among the charges lodged against the pair is second-degree assault, which is eligible for bail under New York State’s controversial bail reform measures — and both have pending cases hanging over them.

Sanchez has an open case in the Bronx on petty larceny and possession of stolen property charges from an incident in October, and a pending Queens case on the same charges from May.

Those charges are also not bail eligible in the state.

Moreira was charged with assault and possession of stolen property in Queens in January, and was also released without bail — even though the first-degree assault charge against him is eligible for bail.

According to sources, Moreira may face additional charges for allegedly throwing urine at a cop after the Tuesday afternoon subway arrest.

Additional reporting by Amanda Woods