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LONG ISLAND — Students at three Long Island high schools were warned against protesting during the national anthem, school officials said.

The Diocese of Rockville Centre said students at ​Holy Trinity Diocesan High School, ​McGann-Mercy High School and St. John the Baptist Diocesan High School could face “serious disciplinary action” if they kneel during the anthem before school sporting events.

“This is a reminder that all student athletes and spectators are expected to stand during the playing of the National Anthem at school sporting events, without any gestures of demonstration or protest,” the statement said. “This is long-standing school policy  and applies equally to all participants and audiences at all school events, including those that are not sporting; for example, concerts and plays.”

Former San Francisco 49ers quarterback Colin Kaepernick started the form of protest. Hundreds of other athletes have followed his lead.

The issue has gained widespread attention in the wake of President Trump’s critical remarks about players who don’t stand for the anthem. Trump suggested team owners fire players who refuse to stand during the anthem.

People outside the professional sports world have followed Kaepernick’s lead, including a team of 8-year-old football players.