A third of young people have been involved in a "sexualised" online chat, it added.
One 16-year-old, Alex - not her real name - told Newsbeat life became "hell" after she sent her boyfriend a private "sext" video during a night out.
"He left his phone on the side and his mates got hold of it," she said.
The video was distributed around her school and neighbouring colleges and Alex said she was forced to take time off from her studies.
"When I came back it was like hell," she said. "I've never really been able to live it down. People I have never met still shout and swear at me. It's been the biggest lesson, I would just never do it again."
But another teenager told Newsbeat there was nothing wrong with "sexting".
"I send pics all the time. If you are comfortable with your body, then why not?", she said.
In theory, teenagers could be arrested for taking naked photos of themselves or their boyfriends or girlfriends.
Although it is legal to have sex at 16 under British law, it is illegal to take, hold or share "indecent" photos of anyone under 18.
Helen Penn said in practice it is unlikely the British police would get involved in a consensual case of "sexting" because it would not be in the public interest, although there have been a number of charges brought in both Australia and the US.
But if grooming or sexual abuse of a minor was discovered, there would probably be a prosecution.
"When the police look at this kind of offence, they are going to take it in context.
"So if it is two 17-year-olds and they are in a consensual relationship, they will probably not prosecute those people.
"But it will look at people that sent on these images and hold lots of them for ill intent."