Young women are under increasing pressure to live up to the ideals of boys who want their girlfriends to look like porn stars, it was suggested on Monday.
Girls are attempting to copy ever-more extreme body images, with some even ranking each other on how they look each day, according to Helen Porter, a member of the Association of Teachers and Lecturers (ATL).
She said it is "deplorable" that teenagers desire unattainable body types, and suggested that they need to be taught that for most people, this is not achievable.
Ms Porter is due to propose two motions at the ATL's annual conference next week calling for action on promoting health body images and the effect of pornography on pupils.
"In the last five years there's been an increase in girls becoming increasingly concerned at body image, comparing themselves to celebrities and unrealistic ideals portrayed by the media," she said.
"The desirable body image has become more extreme, they are all trying to have a narrow waist, long, slim legs and big boobs. That's the desirable thing."
Ms Porter suggested that girls begin to feel the pressure around the age of 13 and 14.
"It's when they show more interest in boys and are worried about what boys like and expect."
She warned that young people do not realise that these types of extreme body images are only achievable by a small proportion of the population.
"We need to educate them to realise that for most people this is not achievable."
Ms Porter said she had heard of girls who "mark each other out of 10 each day on how they look, and how they look in their clothes".
She said: "We need to promote a positive body image so they don't feel so much that they need to judge others. They need to be able to accept themselves for who they are.
"Girls and boys are viewing more pornographic images. The boys are seeing these porn stars and saying 'I'd like to have a girlfriend that looks like that'."
Ms Porter's resolution on pornography calls for the conference to express concern about "the effect that readily available pornography is having on children".
It asks for the ATL's executive committee to conduct a survey to investigate the problem, and to produce guidance for members on how to deal with "sexting", the consequences of pupils' exposure to pornography and "how to proceed when discovering sexual activity taking place on education premises".
The second motion says the ATL should express concern about "the way in which the promotion of ideal and often unrealistic body images in the media reduces children's confidence in their own shape" as this can lead to children developing unhealthy behaviour.
Ms Porter, who is also teacher at St Gabriel's School in Newbury, Berkshire, says that the ATL should back a new all-party parliamentary campaign on body image, to help educate children about healthy body types.