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.
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.
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.
The report spoke to eight focus groups of children aged eight to 12, and found some saying:
- "If I got 150 likes, I'd be like, 'that's pretty cool, it means they like you'" – Aaron, 11
- "I just edit my photos to make sure I look nice" – Annie, 11
- "My mum takes pictures of me on Snapchat… I don't like it when your friends and family take a picture of you when you don't want them to" – Hassan, eight
- "I saw a pretty girl and everything she has I want, my aim is to be like her" – Bridie, 11