"I downright refuse to do PE," admits 16-year-old Josh who believes he is fat and fears bullying.
The teenager, from Renfrew near Glasgow, is so worried about his appearance that he photoshops all of his online profile pictures.
Almost a third of 2,000 UK teenagers, polled for a body confidence campaign, avoid activities like PE because of fears about their looks.
The Be Real Campaign says schools have a key role in combating body anxiety.
Fears about they way they look are "having a profoundly negative effect on a significant number of young people in the UK", says the report.
Some young people are able to reject the pressure to look perfect but "those who cannot often suffer in silence, too afraid to share their insecurities with others... a sense of hopelessness often dominates these individuals", it warns.
Josh told the researchers that he had already had to leave a school "because of bullying and it was all because I was fat".
He said the trend for plus size models had not taken off in men's fashion, adding: "I genuinely don't know one big male model."
And he believes a lot of the pressure to conform to a certain type stems from images in the media "celebrities, plastic surgery, the Kardashians".
"See the guys in my year, a lot of them are like tanks with six packs.
"A lot of guys get their eyebrows done, including me and I photoshop every one of my profile pictures. I get rid of my spots, I get rid of my double chin and it genuinely takes me 25 minutes to make one."
According to the research more than half (52%) of 11 to 16-year-olds regularly worry about their looks - 60% of girls and 43% of boys.
- 79% said their looks were important to them
- 63% said others' opinions of their looks were important
- 36% said they would do whatever it took to look good
- 57% would consider dieting to change their looks
- 10% would consider plastic surgery.
Despite these pressures, fewer than half (48%) of the young people surveyed said they had discussed body confidence in lessons.
- 43% said they were most likely to seek support from parents
- 32% said they would go to their friends
- 11% of boys felt unable to discuss the issue with friends, compared with 5% of girls.
Be Real has launched a toolkit to help schools promote pupils' body confidence.
"Evidence shows that schools are uniquely placed to support young people to hold positive discussions around body image with their peers and help reduce the negative impact low body confidence can have," said Denise Hatton, YMCA chief executive for England and Wales.
Julie Hunter, assistant head teacher at Bradon Forest School in Wiltshire, said the toolkit could help teachers "to use accurate language so they're confident when delivering these lessons".
"Equally it is vital that we make students aware of this issue," Ms Hunter added.