Students across the country are increasingly facing a form of bullying known as “lunch shaming,” where classmates secretly photograph them while eating and share the images online, mental health experts warned.

Experts say the shaming tactic can damage students’ self-esteem, worsen body-image concerns and make students avoid cafeterias altogether, the Wall Street Journal reports.

“The shaming tends to fall into two categories: the ugly mouthful and the lonely eater,” the newspaper reported.

School cafeterias are especially vulnerable to bullying because they typically have less adult supervision than classrooms, Catherine Bradshaw, a University of Virginia researcher who studies bullying and child mental health, told the outlet. Her research of about 25,000 students found that 14 percent of elementary students and 18 percent of middle and high school students reported being bullied in the cafeteria in the past month.

Bradshaw and other experts say the rise of smartphones has transformed a long-standing issue of cafeteria teasing into something more persistent and public. “Catching people doing embarrassing things has been going on for some time, and this is the more recent iteration of that,” she said.

Christian Okafor, a high school senior in San Diego, told the outlet that he has been targeted by the practice dozens of times, estimating that classmates have taken and shared unflattering photos of him while eating between 30 and 40 times.

“It makes you feel self-conscious and like you have to hide while eating,” Okafor said.

Okafor said the repeated lunch shaming led him to seek out secluded spots on his school’s outdoor campus where he could eat without drawing attention. While he eventually learned to ignore the behavior, he said he still regularly sees other students being targeted. He has not reported the incidents to school administrators, however, because he doubts there is much they could do to stop it.

Nihar Patel, a recent graduate from Fairfield, Ohio, told the WSJ that he stopped eating in the cafeteria after a friend became a victim. He said a group of students photographed his friend eating and posted it to a school Snapchat page, prompting him and his friends to begin spending lunch periods in a teacher’s classroom instead.

“She stopped eating at school,” Patel said of his victimized friend. “I know she’s always had body issues.”

Schools are responding with cellphone restrictions and other anti-bullying efforts, but educators say digital harassment is evolving faster than policy, per the WSJ. Some districts have introduced full-day phone bans, while others limit phone use only during class. Early evidence suggests these restrictions can reduce disciplinary issues and improve academic performance, the outlet reports.

In Aberdeen, Washington, a full-day phone ban led to a noticeable increase in middle school students eating school lunches again, as students no longer feared being photographed while eating.

“They were getting their lunch because they no longer feared having their photos taken,” co-superintendent Traci Sandstrom told the WSJ.

In some cases, students say phone bans have reduced lunch-shaming posts, but the behavior has not disappeared entirely. Patel said his school’s ban significantly cut down on such incidents, but some students simply shifted their focus to other forms of online ridicule, including social media pages dedicated to mocking peers who are bad drivers.

Even as schools try to limit phone use, subtle and hidden forms of online bullying remain difficult to eliminate.

“It’s the subtle hidden bullying that’s the worst,” Kaplan Jørgensen, an associate professor of social psychology at University College Copenhagen, told the outlet.