Tackling a hard-core of persistent truants is the key to cutting absenteeism in schools, says government-commissioned research.
The truancy rate has not fallen, despite a series of initiatives
Efforts to cut national truancy rates have struggled - but this research says absence figures are pushed up by a small number of pupils.
Researchers found that 2% of pupils accounted for almost half of absences.
In a study of over 450 schools, researchers found some individual pupils missing a half term each year.
Despite a series of anti-truancy initiatives, including jailing parents, pupil absenteeism has proved a difficult problem to tackle.
A report from the National Audit Office in February showed that despite initiatives costing £885m, the truancy rate had not fallen since the government took office in 1997.
The three-year study from the National Foundation for Educational Research looked at the attendance records of 100,000 secondary school pupils in inner-city schools.
And it has found that small concentrations of truants account for a large proportion of absenteeism figures - rather than a pattern of large numbers of pupils infrequently playing truant.
Almost two thirds of pupils in the survey had no record of unauthorised absences.
However, about one in 20 pupils had unauthorised absences equivalent to missing two weeks of school.
A smaller group of pupils - about one in a hundred - were missing much longer periods of school, representing a half term or more each year.
The survey considered the impact of truancy on pupils' achievement - and found that it had a particularly negative impact on boys.
"Schools and local education authorities are now seizing the tools we have given them to improve school attendance and crack down on the very small number of pupils which account for almost half of the nation's truancy," said a Department for Education and Skills spokesperson.
Shadow Education Secretary, David Cameron, said the government had "failed to get any real grip on the problem".
"They claim to take truancy seriously, but as yet they have very little to show for it. It is doubly worrying to see these figures are focused particularly on deprived areas."