Around two fifths of them said they would like to be thinner, with a fifth of them admitting to feeling under pressure to be slim.
One girl said: "When you become a teenager, you're developing more, you're getting spots and you're feeling a bit iffy about your weight."
Boys were more relaxed about their appearance - and less bothered about what their mates thought.
"I'm my own person and I have my own style and that's it really," said one lad.
And while girls said their style was influenced by pop stars and magazines, boys were more likely to copy their sports heroes.
On the positive side, only 2% of the kids questioned said they had suffered from an eating disorder.
And nearly two thirds of them said they were happy with their look.
Tune in to Newsround at 5.20pm on BBC One for more reports on body image