NEW YORK >> On a September morning two years ago, Abel Cedeno put on a pink Kylie Minogue shirt, and then tossed his books and inhaler into his backpack.

Just before leaving for school that day, he took a switchblade knife he had bought online from the top of his dresser and placed it in his pocket.

Within a few hours, the then 18-year-old senior had fatally stabbed Matthew McCree, 15, and permanently maimed Ariane LaBoy, 16, in a history class at the Urban Assembly School for Wildlife Conservation in the Bronx.

A state judge today found Cedeno guilty of manslaughter, assault and criminal possession of an illegal knife, rejecting Cedeno’s claim he had acted in self-defense after McCree had punched him. Cedeno faces up to 50 years in prison for the first two charges.

Cedeno had waived his right to a jury and had put his fate in the hands of Justice Michael A. Gross.

In eight days of testimony stretched over a three-week bench trial in state Supreme Court in the Bronx, more than 20 witnesses, many of them students, testified regarding what Officer Oliva Carvajal called in testimony “a scene out of a massacre” and what prosecutors said took just 15 seconds to unfold.

By the time Carvajal of the New York Police Department had responded to the emergency call, blood covered the walls of the fifth-floor classroom and McCree shuddered for breath outside the door, while people inside the classroom crowded around Ariane, tapping his face to keep him from fading away.

Cedeno, now 19, told Gross last week that the knife, which he purchased on Amazon for $44.89, was meant only to “deter” students he said had bullied him for years because he is gay.

On that morning in 2017, students in the back of the classroom were throwing crumpled paper and broken pencils, which Cedeno said he thought were aimed at him. Cedeno said he confronted the students, which led McCree to move toward him. Fearing for his life, Cedeno said, he grabbed his knife, flicked open the blade and “just waited.”

During closing arguments last week, Nancy Borko, the lead prosecutor, said Cedeno was looking for a fight with “his trusty new knife.”

McCree’s death, the first homicide in a New York City school in two decades, prompted protests from parents over the lack of metal detectors at a school with a history of violent incidents. Members of the LGBTQ community argued the school should have taken action against those who had bullied a gay student.

Christopher Lynn, one of the defense lawyers, said in closing arguments that Cedeno “did not attack anyone that day.” Instead, he had tried to de-escalate the onslaught, as he often did, by leaving class in the midst of a pummeling. But when he returned, Lynn said his client got hit with debris again. “He was attacked and never the aggressor,” Lynn said.

Lynn and Robert J. Feldman built a defense on the premise that Cedeno had suffered unchecked taunts in the classroom, which drove him to take extreme action.

The two lawyers took the case, pro bono, at the behest of Rubén Díaz Sr., the City Council member who has been criticized for making homophobic remarks but who had bailed Cedeno out of jail.

Feldman framed the proceedings as “a gay-pride trial.” He and Cedeno often wore matching rainbow heart pins clipped to their lapels. Tom Shanahan, who is representing Cedeno in a lawsuit against the Department of Education and later joined the defense table, regularly wore rainbow-colored accessories.

The defense’s decision to forgo a jury trial drew criticism. Feldman explained before the trial that he did not believe Cedeno could get a fair jury trial in the Bronx, where he said anti-gay sentiment among African Americans was high.

Further, Feldman said, Cedeno, whose family is from Puerto Rico, could not count on Hispanic support.

But ultimately, the judge determined that Cedeno, regardless of his sexual orientation and the history of unchecked school bullying, could not provide an account of what happened that day that coincided with many other witnesses’ accounts. All the other students who testified said Cedeno initiated the fight, and that he was the only person using homophobic slurs that day.

The case sparked strong emotions, as well as extensive litigation — three lawsuits have been filed against the Department of Education by both sides.

The two sides regularly held news conferences after each day’s proceedings. During one, defense lawyers suggested that the victims had been gang members, even though they never provided evidence to prove it.

In closing arguments, Lynn spoke about the violent reputations of both victims.

On Thursday, the defense submitted a photograph of an African American teenager who was wearing a black bandanna and identified him as McCree. They said the bandanna indicated an affiliation with a local gang. Shortly after, McCree’s mother, Louna Dennis, took the stand and said the person in the photo was not her son.

“I’m glad I got the chance to say something, to make corrections about some of their errors,” she said after taking the stand. “They perpetrated a lot of lies.”

Later, outside the courthouse, Dennis labeled the defense’s arguments as “straight racist.”

Sentencing is set for Sept. 10.