Teachers tend to regard short boys as less bright than their taller classmates, researchers have found.
American researchers Julia Smith and Nancy Niemi say that teachers underestimate small boys' ability in all curricular areas.
Previous studies have shown that overweight children are perceived negatively, particularly girls. But few studies have looked at how teachers are influenced by boys' physical appearance.
The Smith and Niemi study, which was presented to the American Educational Research Association conference in San Diego, surveyed almost 9,000 kindergarten boys in 963 schools.
The researchers classified 15 per cent of the boys as short because they were at least two inches shorter than the national average for their age - just under 3ft 9in.
Every child was given tests to measure their reading, maths and general knowledge and each teacher was asked to predict how well the child would perform. The test scores were then compared with the teachers' assessments.
Julia Smith, of Oakland university, Michigan, and Nancy Niemi of Nazareth college, New York State, also took into account four factors that could determine the child's height and views of their ability: ethnic background, socio-economic status, age and birth weight.
This established that there was a mismatch between the boys' ability and teachers' perceptions in all three subjects that was "large enough to be significant as well as noticed by children".
"In general, even after adjusting for related background characteristics, teachers systematically underestimate the performance of smaller boys in all curricular areas, putting them at risk of being considered in need of remediation, when in fact their skills levels are quite adequate," the researchers said.