Nearly 1 in 5 students in U.S. middle and high schools — 18.5 percent — said they have received sexually explicit images or videos on their phones or computers, a practice known as sexting, a study says. Those numbers do not include adolescents who have received sexually explicit or suggestive text messages or emails without an image, another form of sexting. The study’s data, restricted to video and photo sexting, was compiled from a nationally representative sample of 5,593 students ages 12 to 17 and published online this month in the journal Archives of Sexual Behavior. In addition to sexts received, about 13 percent of adolescents say they have sent a video or photo sext, the researchers found. Most of them (64 percent) sent these images or videos after being asked to do so by a current boyfriend or girlfriend. About a third of those who sent or received explicit images say they did so just once. About 2 to 3 percent say they sent or received a video or photo sext “many times.” The researchers found that boys said they sexted more often than girls, and older adolescents more than younger ones. The study found no racial differences among those receiving or sending sexts. Based on their data, the researchers said that “the vast majority of students were not participating in sexting,” but that the “real risks associated with sexting, including reputational damage and possible legal consequences,” make it important for adults to have an open and honest dialogue with adolescents about sexting. The American Academy of Pediatrics urges parents to start that conversation as soon as a child is old enough to have a cellphone, and not wait until a problem develops.