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OWATONNA, Minn. — “I knew it wasn’t O.K.,” Kloey, 16, said. “I knew that for sure.”

Late one Saturday night in February in Owatonna, Minn., Kloey posted a selfie on Snapchat with two of her friends. Kloey stuck out her tongue, Candace pursed her lips and Grace wore a wide-eyed grin. While singing along to a rap song in Kloey’s car, Grace, who is white, used a hateful racial slur for what she said was the very first time. Kloey, also white, posted the photo on Snapchat to commemorate the occasion, spelling out the slur in the caption.

The post spread quickly among Owatonna High School’s small population of black students, who had felt for years that racism had been allowed to quietly fester in their school. Not again, they said to each other in anger.

Teenagers flirt on social media. They pour out their souls. And all too often, in an era of viral videos, they show off their intolerance when it comes to race. High school students have been captured flashing the Nazi salute and singing Ku Klux Klan-themed Christmas songs. Teachers have dressed up as a border wall for Halloween and asked their black students to participate in mock slave auctions. The fallout from such episodes often looks the same: online apologies and outrage, and then everyone involved moves on.