Schools should play a bigger role in preparing children for social media's emotional demands as they move from primary to secondary school, England's children's commissioner says.
Anne Longfield said she was worried many pupils at that stage became anxious about their identity and craved likes and comments for validation.
Her study said children aged eight to 12 found it hard to manage the impact.
The government said it was working with schools on online safety education.
The report into the effects of social media on eight to 12-year-olds claimed many children were over-dependent on "likes" and comments for social validation.
It said children approach a "cliff-edge" as they move from primary to secondary school, when social media becomes more important in their lives.
The report spoke to 32 children in eight focus groups, aged eight to 12, and found some saying:
Ms Longfield called on schools and parents to prepare children emotionally for the "significant risks" of social media as they move schools and meet new classmates - many of whom have their own phones.
"It's really when they hit secondary school that all of these things come together," she told BBC News.
"They find themselves chasing likes, chasing validation, being very anxious about their appearance online and offline and feeling that they can't disconnect - because that will be seen as socially damaging."
Although most social media platforms have a minimum age limit of 13, the report said three-quarters of children aged 10 to 12 already had accounts.
'Avalanche of pressure'Ms Longfield said she was surprised at how children viewed social media differently as they grew older.
Younger children use social media for "fun family reasons" but it becomes an "avalanche of pressure" by the time they get to secondary school age, she said.
Parent Trevor said his 12-year-old twin daughters had moved schools as a result of the pressure from social media, but admits they "can't walk away" from it.
He told BBC Radio 5 Live: "I can't say to them 'you can't use that', when I use it".
He said teachers lacked the skills to educate children and said the approach by politicians was "disappointing".
Izzy, a 17-year-old pupil from Bristol, said she was "judged at every corner" on social media, after first seeing the platforms being used by her older cousins.
"Being exposed to all this adult stuff happens very quickly," she told 5 Live.
'Emotional rollercoaster'Ms Longfield said social media provided "great benefits" to children but was also exposing them to "significant risks emotionally".
She called on the government to introduce compulsory digital literacy and online resilience lessons for year six and seven pupils, so that they learn about the "emotional side of social media".
Parents should also prepare their children, she said, by "helping their children navigate the emotional rollercoaster" of the negative aspects of social media.