Nearly 40,000 children aged seven to 16 were surveyed by educational charity London Grid for Learning about their online habits, in what is thought to be the largest survey of its kind.
Overall, one in eight pupils said they had video chatted with someone they had not met in person, with one in 10 of those saying they had been asked to “change or take off clothes” during that chat.
The youngest pupils surveyed, aged seven to eight, were just as likely to report being asked to undress as those in the first four years of secondary school, aged from 11 to 14.
One 12-year-old girl said: “A girl from my primary sent half naked pictures, because it’s ‘what everyone does’.”
A nine-year-old boy told the report: “My sister put a video of me naked onto Facebook… a woman sent me pictures of herself naked.”
Mark Bentley of the London Grid for Learning said: “Two things that really stuck out were mental health and live streaming.
“Self-harm is an issue which stuck out. We found it is a new tactic of bullying, with some children being sent instructions on how to self-harm. One in six said they had seen something which encouraged self-harm.”
He said one in 10 children who chat with someone they haven’t met have been asked to undress and the same is true for one in 20 who live stream.
He added: “There are many positives to lots of apps though, with children saying they want to use them and talk to their parents more about them.
“It doesn’t matter if you don’t understand the latest app as long as you understand the procedure and that’s the message we want to give parents.
“We want to encourage them to have a dialogue with their children.”
Children also reported seeing high levels of violence online, including some who said they had witnessed people self-harming, seen beheading videos, or watched something that glorified suicide.
Boys are twice as likely as girls to meet up with someone in the real world who they have met online, and are more likely to see violent images or videos.
Half of those who chat to people they meet on games go on to talk to them on other platforms.
But many of those exercise some precaution, with 81% telling someone or taking someone with them, when they go to meet someone for the first time.
Children were asked for their best and worst experiences online, and 61% confessed they had never spoken to anyone about their worst experiences on the internet. Those who did were far more likely to speak to family or friends than call a helpline.
However, many children were open about the positives of social media, using it to keep in touch with friends in different schools, or to encourage friends who are feeling upset or down.
Of the 39,824 surveyed, 54% were from primary schools, and 46% from secondary schools, with 52% girls and 48% boys.