Thousands of pupils in England's inner cities skip school for up to two weeks every year, Government research says.
Hundreds of pupils were truant for the equivalent of a whole term or more, according to the study conducted by the National Foundation for Educational Research.
Analysing figures for more than 100 , 000 pupils, researchers found that boys who skipped lessons fell further behind with their school grades than girls.
The three-year study followed figures showing that more than 1,100 fines had been issued to parents for failing to sent their children to school since September 2004.
The latest study focused on 454 secondary schools in England's more deprived urban areas that have been targeted for extra government help.
These included schools in Birmingham, inner London, Manchester and Leeds.
Most pupils - 65 per cent - had no recorded periods of "unauthorised absence".
But more than 1,000 pupils in these schools were truant for the equivalent of half a term every year.
Being out of school either truanting, on holiday or sick for more than five weeks a year cut a pupil's chances of getting one good GCSE at grade C or better by 30 per cent.
"This might suggest that the impact of poor attendance might be greater for lower attaining pupils than for higher attainers.
The research found that several hundred pupils were truant for one third of the school year - a term.
About one per cent - 1,000 pupils - were absent with authorisation from their schools for a whole term or more.
The report said 25.2 per cent of the pupils in the study were absent without authorisation for up to a week.
And for five per cent, or about 5,000 pupils, " unauthorised absence amounted to up to two weeks" a year.
A spokesman for the Department for Education and Skills said: "The majority of incidents of unauthorised absence in the participating schools in this study were accounted for by a minority of