Overweight girls rely on teachers to look beyond cultural stereotypes and respect the brains within, say Julia Smith of Oakland University, Michigan, and Nancy Niemi of Nazareth College, New York state.
But teachers are actually biased against them. They underestimate their performance, especially in general knowledge. "The more overweight the girl, the less intellectually capable her teacher rates her," say Smith and Niemi.
They used findings for a representative sample of nearly 10,000 girls from a national longitudinal study of children starting kindergarten in 1998.
They compared the results of tests they took in reading, mathematics and general knowledge with the teachers' assessment of their performance.
Staff, who are overwhelmingly female, consistently rated the performance of overweight girls lower than the results they actually achieved.
The bias was least important in reading, where the difference between assessment and test result was about 2.5 percentile points. But in maths, the difference was five points and in general knowledge 10 - where the teacher rated the child's general knowledge as in the top 20 per cent nationally, the test showed it was actually in the top 10 per cent.
"If a girl perceives that 'my teacher thinks I'm stupid', the chance that the girl will live down to this perception is all too possible," they say.
"Coupled with her disadvantage of being overweight in a culture that prizes extreme thinness in women, the overweight girl may no longer have academic study as her refuge."
Psychotherapist Susie Orbach, author of Fat is a Feminist Issue, reacted with "horror" to the findings. She hoped the study would make teachers aware of their prejudice, so that they would "bend over backwards to overcome it".
The researchers, who presented their paper to the recent American Educational Research Association conference in Chicago, found an eveb greater bias against short boys.