Children as young as four are suffering from low self-esteem due to their physique, according to teachers.
Boys as well as girls say they are unhappy that they do not measure up to media images of the perfect body, the Association of Teachers and Lecturers has warned.
Eating disorders are increasingly prevalent among younger age groups, with children refusing to eat foods such as cheese for fear of becoming fat, the survey of almost 700 primary and secondary school teachers found.
The union said 78 percent of its members thought girls suffered low self-esteem over their body, while 51 percent thought boys suffered a similar dent to their confidence, the poll found.
Pressure to attain the perfect body led to anxiety among female pupils, according to 59 percent of staff. Male pupils increasingly felt the same way, said 30 percent of those polled.
Girls were "ultra-sensitive" to comments about their appearance, said 55 per cent of those questioned, while 27 percent said boys felt the same.
Girls were likely to diet to improve their image, said 54 percent of staff, while boys were prone to start severe exercise regimes, said 30 percent.
The poll found 84 percent of teachers said girls felt under pressure to have a certain body image, with two-thirds (66 percent) saying boys faced the same pressure. That pressure is mounting, with 63 percent saying the problem had got worse over the past decade.
Almost half of teachers (46 percent) said that girls used clothes to disguise their body shape. Over half (51 percent) said girls chose outfits to "flaunt" their bodies, and 29 percent claimed that girls used sunbeds or spray tans.
More than a fifth (22 percent) said boys were using protein shakes to improve their physique.
Almost half (48 percent) believed girls had developed eating disorders such as anorexia or bulimia due to pressure to live up to a certain body image.
The media, including online social media, were largely to blame for the problems, with 94 percent of techers saying the media was the biggest influence on female pupils and 68 percent reporting a similarly negative influence on boys.
Peers of the same sex were identified as a cause of pressure on female pupils to look good by 83 percent of respondents and on male pupils by 54 percent.
The union will discuss the issue of body image among pupils at its annual conference in Liverpool next week.
The All-Party Parliamentary Group on Body Image last year recommended all children take part in body image classes.
Last November, plans for a UK qualification in body image were unveiled, with lessons covering body image in the media, self-esteem, diet and exercise for classes aged 11 to 14.
The qualification is to be rolled out in secondary schools from next year.
ATL general secretary Dr Mary Bousted said: "Young people are under tremendous pressure to have or maintain often unrealistic body images portrayed in the media.
"ATL members report that this not only impacts on female pupils but increasingly leads to low self-esteem, lack of confidence and anxiety in male pupils too.
"With academic and other social pressures young people already have enough to deal with."