The victim of an online "bait out" group has told how she misses school and "cries all the time" - two years after her private images were shared.
The Kent schoolgirl told BBC South East Today she was 14 when she sent sexual images of herself to a boy, who posted them online for her peers to see.
The BBC found more than 50 such groups on social media.
Sharing indecent images of a child is illegal even if the person doing so is a child, said Kent Police.
Speaking anonymously, the victim, now 16, said she sent pictures of herself with underwear on to a boy.
She found out the images had been shared when some of her friends made fun of her.
"They looked at me and they whispered to their friends and I knew it was all about me," she said.
The teenager said there were times when she still could not attend school.
"I can't concentrate and I just cry all the time - not all the time, but when I remember," she said.
'Particularly targeted at girls'
"Bait out" pages are online groups or pages that invite users to share nude images, videos or sexual gossip about others, according to Childnet.
Users are often only allowed access to the group or page once they have shared such content.
It can also involve videos being shared on public platforms such as YouTube where people are "named and shamed".
Research with 1,559 teenagers aged 13 to 17 conducted by Project deSHAME found 39% had witnessed people setting up a "bait out" page in the past year for people in their school.
Project manager Maithreyi Rajeshkumar said technology played an important role in young people's lives but also allowed for "new forms of harassment".
"This shaming culture is particularly targeted at girls, who can face ongoing harassment and bullying," she added.
Figures released by Kent Police show crime reports for sexual messages have more than tripled over two years, from 34 in 2015 to 117 in 2017.
Det Ch Supt Tom Richards said: "It has had very significant impact for some individuals - huge trauma. It can be real bullying scenarios, very embarrassing, very humiliating.
"With previous generations, the technology wasn't there and attitudes were different. Today many people think it's perfectly reasonable.
"Most people have got mobile phones now, most people have social media, most phones have fantastic cameras on them now and it's just a lot easier than it used to be."
The BBC found more than 50 "bait groups" operating on Instagram.
Instagram said the accounts had been investigated and those active removed for violating guidelines.
It said keeping the platform a safe and positive place was its top priority.
See more on this story on South East Today on BBC One at 13:30 BST and 18:30 and later on the iPlayer.