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Abel Cedeno says he was bullied for years in school by other students because of his perceived sexual orientation. Then, in September 2017, he sent a message to a friend suggesting he was willing to defend himself: a video of himself, flicking open a switchblade. A caption asked why people thought he was soft.

Eleven days later, Mr. Cedeno stabbed a fellow student to death at his Bronx high school in what his lawyers have called an act of self-defense against two bullies but prosecutors say was a reckless act of violence.

His trial on manslaughter and assault charges, which began with opening statements on Friday, is expected to focus attention on the issue of bullying directed at L.G.B.T. teenagers.

Mr. Cedeno, 19, plans to testify in his own defense. He has waived his right to a jury and put his fate in the hands of Justice Michael A. Gross in State Supreme Court in the Bronx.