Pupils caught staring into the middle distance, looking at the ceiling or closing their eyes are as likely to be struggling with a complex problem as daydreaming.
They are also more likely to come up with the right answer than if they behaved in a more overtly attentive manner.
Teachers should instead encourage pupils to look away from them while thinking as it will improve the quality of their response, a study by Gwyneth Doherty-Sneddon of Stirling university found.
Forcing children to look at their teacher while they think breaks their concentration, the study says.
Writing in The Psychologist, Dr Doherty-Sneddon said: "There is a tendency in many cultures ... to interpret looking away as a sign of disengagement or lack of interest."
She said her research showed that primary-aged pupils looked away to help them concentrate on difficult material.
Dr Doherty-Sneddon believes that observing a child's pattern of gaze could help teachers judge a pupil's knowledge and development.
Younger children are more likely to look at a questioner because they rely more on visual clues to understand what is wanted and are more likely to attempt to get help from the adult rather than work things out for themselves.