Teachers know of their existence already. They are the pupils who yawn and glaze over during lessons. They are the ones who wriggle, who fidget, who are chronically unable to concentrate.
But new research has confirmed their existence: up to a fifth of school pupils regularly stay out after 9pm, without their parents knowing where they are. And significantly more say they stay out late each month.
Maria Iacovou, from the University of Essex, analysed data from more than 2,000 children aged between 10 and 15. Her research forms part of the long-term Understanding Society study, funded by the Economic and Social Research Council.
Teachers have often expressed concern about how late children and teenagers stay out. But, Iacovou writes in her report: "This issue has become particularly relevant in contemporary Britain, against the backdrop of the riots of 2011, with poor parenting being blamed in some quarters for the disturbances, and with pronouncements by Parliament and regional police forces that parents should make sure they know where their children are at night."
In the new study, one in five boys and one in six girls report having stayed out after 9pm at least once over the previous month, without their parents knowing their whereabouts.
Family income appears to have little effect on this aspect of pupils' behaviour, although children living in social housing are more inclined to stay out late. So, too, do those from single-parent families. Asian teenagers are most likely to be home early, with white and African or Caribbean teenagers more likely to stay out beyond 9pm.
Only a low percentage - 4 per cent of boys and 2 per cent of girls - report having stayed out late at least 10 times over the previous month.
Unsurprisingly, however, when the figures are broken down this tendency increases dramatically among 15-year-old pupils. A quarter of the girls and a third of the boys say that they are out late more than twice a week without their parents knowing where they are. In fact, boys of all ages are more likely than girls to stay out late without informing their parents.
But, Iacovou says: "The question is whether staying out late without parents knowing is a problem behaviour, or simply a manifestation of the fact that children become more independent, and that their parents trust them more to make their own decisions as they reach their mid-teens."
Sleep deprivation alone can mean that pupils are restless and poorly behaved in school. Iacovou's study found that boys who regularly stay out late are at greater risk of conduct problems than their classmates: almost one in five has behavioural problems. Girls who stay out at night are more likely to be hyperactive and to suffer from low self-esteem.
Iacovou also found that staying out late is associated with other potentially problematic behaviour patterns. Fifteen-year-olds who stay out late without their parents' knowledge are much more likely to smoke, drink and take drugs than their stay-at-home classmates.
Indeed, two-thirds of 15-year-old girls who stay out late had drunk alcohol more than once in the previous month, compared with only a quarter of stay-at-home girls. And half of the girls who stay out frequently smoke, compared with 18 per cent of those who are home by 9pm.
Meanwhile, boys who frequently stay out late are five times as likely to use cannabis as those who do not stay out late.
"Regularly staying out late without telling their parents where they are is symptomatic of a young person with underlying problems," Iacovou says.
Understanding Society report: http:bit.lyAAu5aB
Maria Iacovou, Institute for Social and Economic Research, University of Essex: www.iser.essex.ac.uk
Economic and Social Research Council:
21% of boys aged 10-15 had stayed out late without their parents' knowledge within the previous month
15% of girls aged 10-15 had done the same thing
4% of boys aged 10-15 had stayed out late more than 10 times within the previous month
2% of girls aged 10-15 had done the same thing
36% of boys aged 15 had stayed out late more than 10 times within the previous month
24% of girls aged 15 had done the same thing
64% of girls who stay out often consume alcohol more than once a month, compared with 25 per cent of girls who had not stayed out
51% of girls who stay out frequently smoke, compared with 18 per cent of girls who had not stayed out.