The racist social-media posts were originally shared only among friends — in text messages and a Google document. But someone took screenshots, which led Harvard University to revoke an offer of admission to a Parkland high school survivor.

The decision announced Monday serves as a reminder to aspiring college students and all young people that online comments, even those considered private, can resurface and be used against them.

It's relatively unusual for colleges to rescind admission offers. When they do, it's more often for a slip in academic performance or disciplinary issues than social media posts. But experts say it's not uncommon for offers of admission to be jeopardized by the emergence of damaging communications, sometimes because of people motivated by competition or jealousy.

Parents of students rejected by a college on many occasions have reported the social-media posts of other students who were accepted, said Jeff Fuller, former admissions officer at the University of Houston and now director of college counseling at Strake Jesuit College Preparatory, also in Houston.

"The unfortunate thing is that this may be your best bud, but tomorrow it might not be," Fuller said. Anything communicated in writing "is subject to somebody using it in a negative way against that individual."

Kyle Kashuv, 18, posted on Twitter that Harvard had revoked his admission over anti-Semitic language and repeated use of a slur referring to black people. He said in an apology that he made the comments when he was 16, before the February 2018 shooting at Marjory Stoneman Douglas High School in Parkland, Florida. He said he made "idiotic comments" and used "callous and inflammatory language in an effort to be as extreme and shocking as possible."

An advocate for gun rights, he said in his announcement on Twitter that political opponents had contacted Harvard and urged the school to cancel his admission over the posts.

Kashuv's posts drew mixed reactions, with some saying he deserved to be forgiven and others saying Harvard made the right decision.

Brad Shear, an attorney based in Baltimore who specializes in social media law , said Kashuv deserved a second chance at Harvard. He said teens "say and do stupid things."

"Once your name is tied to something digitally, that information can be used against you forever," Shear said. But, for some young people, "until they touch the hot stove, they don't learn they can get burned."

In 2017, Harvard also pulled offers from 10 incoming freshmen after they reportedly made racist and sexually offensive comments in a Facebook group.

The National Association for College Admission Counseling found in 2015 that less than a third of its members at more than 1,700 colleges reported rescinding an admissions offer each year. Nearly 70% of those schools said it was because of a dishonest application, while 20% said it was over a disciplinary issue. Social media behavior was not considered a reason to drop a student.

But a 2016 Kaplan Test Prep survey found that 40% of college admissions officers browse social media profiles to learn more about admissions candidates.

"We always encourage students and adults to be careful if what you put out there regardless of what you thought because it could get out in a more formal setting that you may not be intending," said Kent Rinehart, dean of admissions of Marist College in Poughkeepsie, New York.

Diana Graber, founder of Cyber Civics and Cyberwise , classes for students and parents, said she has heard of other examples of students' competitors flagging potentially damaging material online.

Graber writes in her book about an admissions officer at a California university who told her about receiving copies of posts from "fake" social media accounts showing a young woman in "half-naked selfies" and "posts strewn with foul language."

The enclosed note read: "You need to know what this girl is really like; she's not as squeaky-clean as you think."

Young people should stop and think about everything that has their name attached to it, said Laura Tierney, chief executive officer and founder of The Social Institute , based in Durham, North Carolina, which works with schools nationwide to teach students about the safe use of social media.

"Anything you click 'send' on represents your character at the end of the day," Tierney said "Anything can be screen-shotted and shared beyond the people to see it."

Haley Carter, 14, of Mission Viejo, California, has taken the Cyber Civics course for three years.

She heard about Kashuv after hearing of many others. "I understood that it really happens because people really do post things that shouldn't be on the internet, and when people post, it's there forever," she said.

Not only did she learn about real-life examples in the classroom, she also had a real-life example of the dangers of social media from her school. Parents who saw a rap video that students posted on Instagram cited inappropriate language in that video as a reason not to send their students to the school, she said.

"There are parents that are looking at everything, and there are people that are looking at everything," she said. "It was kind of wake-up call that people are looking at things like that."

Waggoner reported from Raleigh, N.C. Associated Press writer Michael Melia contributed from Hartford, Conn.