Karl Wall, from London university’s institute of education, said there was potential for misunderstanding in schools with pupils from many backgrounds.

“There hasn’t been much research,” he told a seminar for London teachers.

“But what there is seems to suggest that we find it easier to recognise the expressions on faces similar to ourselves and who come from the same racial or ethnic groups.”

He said there were some universally recognised gestures such as a smile but they were relatively few.

Teachers also needed to be aware of cultural differences. “If I point directly at you and you are an African-Caribbean youngster you might take that as being very aggressive,” said Dr Wall.

“Telling an African-Caribbean youngster to look you in the face is probably not going to work well because in those cultures looking down is often a sign of respect and looking someone in the eye may actually be challenging them.”

Tony Sewell, an educationist specialising in under-achievement among black boys, said: “There is a danger that we exaggerate cultural misunderstandings. In multi-cultural societies children learn behaviour norms quickly.”

However, pupils who had experienced racism were more likely to pick up subliminal signals from teachers. “Black students have a sharper radar and teachers should be aware of that.”

Dr Wall said the most obvious mistakes teachers made were being too close physically, not giving pupils enough time to answer questions, making physical contact and being unaware that what they did or said might have unintended meanings.

His research in primary classrooms suggested that teachers generally made more positive interpretations of pupils’ expressions than other pupils.

“We assume that the expressions we use with children are received the way we intended them,” he said. “But there is some evidence they may be interpreting them the other way.”