Nick Sandmann has identified himself as the youth standing close to Nathan Phillips, a 64-year-old Native American man, as two marches took place at the same time in Washington last week.
The Indigenous Peoples March in Washington on Friday coincided with the March for Life, which drew thousands of anti-abortion protesters, including a group from Covington Catholic High School in Park Hills.
"I am being called every name in the book, including a racist, and I will not stand for this mob-like character assassination of my family's name," wrote Mr Sandmann, who added that he and his parents have received death threats since video of Friday's confrontation emerged.
Both Mr Sandmann and Mr Phillips say they were trying to defuse tensions that were rising among three groups on a day Washington hosted the marches.
But video of Mr Sandmann standing very close to Mr Phillips, staring and at times smiling at him as Mr Phillips sang and played a drum, gave many who watched it a different impression.
Other students appeared to be laughing at the drummer, and at least one could be seen on video doing a tomahawk chop.
The differing accounts emerged on Sunday as the nation picked apart footage from dozens of mobile phones that recorded the incident amid an increasingly divided political climate fuelled by a partial government shutdown over immigration policy.
Mr Sandmann said he heard no student sing anything beyond school spirit chants, and that he had not even been aware of the Native American group until Mr Phillips approached him.
"The protester everyone has seen in the video began playing his drum as he waded into the crowd, which parted for him. I did not see anyone try to block his path," Mr Sandmann wrote.
"He locked eyes with me and approached me, coming within inches of my face. He played his drum the entire time he was in my face."
Mr Sandmann said one of the Native American protesters yelled at them that they "stole our land" and they should "go back to Europe," but that he never spoke to or interacted with Mr Phillips.
"To be honest, I was startled and confused as to why he had approached me."
He wrote that he "believed that by remaining motionless and calm, I was helping defuse the situation."
"I said a silent prayer that the situation would not get out of hand," he wrote.
He said the incident ended when the buses arrived and his teacher told him it was time to leave.
Though many commenting on the internet were taken back by Mr Sandmann staring at Mr Philipps, the teen said he was "not intentionally making faces at the protester", adding: "I did smile at one point because I wanted him to know that I was not going to become angry, intimidated or be provoked into a larger confrontation".
The Roman Catholic Diocese of Covington apologised for the incident on Saturday, saying "this behaviour is opposed to the Church's teachings on the dignity and respect of the human person".
Mr Sandmann said he has provided a copy of his statement to the diocese and said: "I stand ready and willing to cooperate with any investigation they are conducting."
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