The Association of Teachers and Lecturers claims the promotion of ideal body images is reducing both boys' and girls' confidence in their own bodies.
Last year, the All Party Parliamentary Group on Body Image recommended all children took part in compulsory body image and self-esteem lessons.
The union will discuss the issue of body image among pupils at its annual conference in Liverpool next week.
In a survey of 693 members across the UK, the union found 78% thought girls suffered low self-esteem and 51% thought boys had low confidence in their body image.
In addition, 59% of staff said that worries about body image made female pupils anxious and 30% said it caused anxiety in male pupils.
Some 55% said that girls were "ultra-sensitive" to comments about their appearance and 27% said boys took comments to heart.
Pressure to achieve the perfect body meant girls were likely to go on a diet, according to 54% of staff and boys were prone to start excessive exercise regimes, said 30% of those surveyed.
Asked where the pressure came from, 94% of those surveyed said they believed the media, including television, magazines, music videos, adverts and social media was the biggest influence on female pupils and also on male pupils (68%).
Peers of the same sex were also highlighted as a key cause of pressure on female pupils to look good by 83% of respondents and on male pupils by 54%.
A teacher at a secondary academy in Wandsworth, London, said: "I find that boys who are shorter than their peers are extremely sensitive and manifest either disruptive behaviour or are very withdrawn."
One early years teacher in England said: "I work with four to five-year-olds and some say things like, 'I can't eat cheese, it will make me fat!'"
A teacher at a secondary school in Northern Ireland said: "In my year 11 GCSE English class, the girls all openly admitted to feeling pressure about body image and many of the boys confessed to it too, although they said that usually they wouldn't admit to it affecting them.
A head of department at a secondary school in Exeter said: "Digital and social media are a nightmare with 'anonymous' sites that slate students in the most appalling terms on their appearance, personality and anything judge-able."
"Despite knowing about airbrushing, the girls still feel the need to aspire to unrealistic notions of beauty."
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