Dozens of European studies have helped identify factors, such as poverty, ethnic status, family circumstances, language, type of school, geography and community, that reduce their chances of success.
Peter Evans, of the Organisation for Economic Co-operation and Development, says the research shows Europe's "youth at risk" have more similarities than differences - the most important being poverty.
In most European countries, about 5 per cent of children are thought to need multi-agency support, says Dr Evans, co-author of a newly-published OECD study of state services for disadvantaged children. But although the OECD and other agencies have collated countless statistics, fundamental questions have yet to be answered.
As Professor Carol FitzGibbon, of Durham University has said, researchers have yet to quantify the potential impact of good ante-natal nutrition, for example, or the precise importance of homework schemes.
In Portugal, where 30 per cent of families are considered to be living below the poverty line, researchers have been concentrating on the children in Lisbon's slums. Many of these children - particularly girls - leave school at 12 to look for work to support their families.
Dutch researchers have been assessing the success of the country's educational priority policy aimed at ethnic-minority children and white pupils from poor homes.
Thomas Kellaghan of St Patrick's College, Dublin, says Ireland has also become increasingly aware of the problems of disadvantaged children. He estimates that just over 80,000 Irish schoolchildren - 16 per cent- could be classified as disadvantaged.
Kellaghan and other members of the European Educational Research Association have set up their own network to strengthen research input into policy discussions on what can be done to help youth at risk. But they know there are no easy remedies to the problems.