LEESBURG, VIRGINIA (NYTIMES) - Mr Jimmy Galligan was in history class last school year when his phone buzzed with a message. Once he clicked on it, he found a three-second video of a white classmate looking into the camera and uttering an anti-black racial slur.

The slur, he said, was regularly hurled in classrooms and hallways throughout his years in the Loudoun county school district. He had brought the issue up to teachers and administrators, but, much to his anger and frustration, his complaints had gone nowhere.

So he held on to the video, which was sent to him by a friend, and made a decision that would ricochet across Leesburg, Virginia, a town named for an ancestor of Confederate General Robert E. Lee and whose school system had fought an order to desegregate for more than a decade after the Supreme Court's landmark ruling.

"I wanted to get her where she would understand the severity of that word," Mr Galligan, 18, whose mother is black and father is white, said of the classmate who uttered the slur, Ms Mimi Groves. He tucked the video away, deciding to post it publicly when the time was right.

Ms Groves had originally sent the video, in which she looked into the camera and said, "I can drive", followed by the slur, to a friend on Snapchat in 2016, when she was a freshman and had just got her learner's permit. It later circulated among some students at Heritage High School, which she and Mr Galligan attended, but did not cause much of a stir.

Mr Galligan had not seen the video before receiving it last school year, when he and Ms Groves were seniors. By then, she was a varsity cheer captain who dreamed of attending the University of Tennessee, Knoxville, whose cheer team was the reigning national champion. When she made the team in May, her parents celebrated with a cake and orange balloons, the university's official colour.

The next month, as protests were sweeping the nation after the police killing of Mr George Floyd, Ms Groves, in a public Instagram post, urged people to "protest, donate, sign a petition, rally, do something" in support of the Black Lives Matter movement.

"You have the audacity to post this, after saying the N-word," responded someone whom Ms Groves said she did not know.

Her alarm at the stranger's comment turned to panic as friends began calling, directing her to the source of a brewing social media furore.

Mr Galligan, who had waited until Ms Groves had chosen a college, had publicly posted the video that afternoon. Within hours, it had been shared to Snapchat, TikTok and Twitter, where furious calls mounted for the University of Tennessee to revoke its admission offer.

By that June evening, about a week after Mr Floyd's killing, teenagers across the country had begun leveraging social media to call out their peers for racist behaviour. Some students set up anonymous pages on Instagram devoted to holding classmates accountable, including in Loudoun county.

The consequences were swift. Over the next two days, Ms Groves was removed from the university's cheer team. She then withdrew from the school under pressure from admissions officials, who told her they had received hundreds of e-mail messages and phone calls from outraged alumni, students and the public.

"They're angry, and they want to see some action," an admissions official told Ms Groves and her family, according to a recording of the emotional call reviewed by The New York Times.

Ms Groves was among many incoming freshmen across the country whose admissions offers were revoked by at least a dozen universities after videos emerged on social media of them using racist language.

In one sense, the public shaming of Ms Groves underscores the power of social media to hold people of all ages accountable, with consequences at times including harassment and both online and real-world "cancellation".

But the story behind the backlash also reveals a more complex portrait of behaviour that for generations had gone unchecked in schools in one of the nation's wealthiest counties, where black students said they had long been subjected to ridicule. "Go pick cotton," some said they were told in class by white students.

Ms Groves said the video began as a private Snapchat message to a friend.

"At the time, I didn't understand the severity of the word or the history and context behind it because I was so young," she said in a recent interview, adding that the slur was in "all the songs we listened to, and I'm not using that as an excuse".

Ms Groves, who just turned 19, lives with her parents and two siblings in River Creek, a predominantly white and affluent gated community built around a golf course.

In the months since Mr Galligan posted the video, he has begun his freshman year at Vanguard University in California, and Ms Groves has enrolled in online classes at a nearby community college. Although they had been friendly earlier in high school, they have not spoken about the video or the fallout.

At home, Ms Groves' bedroom is festooned by a collection of cheer trophies, medals and a set of red pompoms - reminders of what could have been. Her despair has given way to resignation.

Since the racial reckoning of the summer, many white teenagers, when posting dance videos to social media, no longer sing along with the slur in rap songs. Instead, they raise a finger to pursed lips. "Small things like that really do make a difference," Mr Galligan said.