I think that we have to save them all - and the time to catch them is when they are toddlers. If they had really small nurture groups at that point, and the parents were involved, they could break the cycle.
I still think that people are misunderstanding.
Firstly they get bogged down with the term. If you go to a maths class you don't argue that you don't like the term and it doesn't exist, but it seems that if they go to a sociology class they don't want to use the accepted term and deny it exists.
The educational underclass are the children who are not able to fully access the curriculum because of home circumstances. If you are a child of drug addicts, who doesn't know where the next meal is coming from and you don't have regular routines you are going to find it much harder to sit on the carpet and concentrate on a maths lesson than the child who had an evening of talking to adults, a cooked meal, regular bedtime with story and breakfast.
Most of the prison population have very poor literacy skills. If money had been spent when they were tiny they probably wouldn't be in prison.
Someone who works in a supermarket or doing nails is not part of the 'educational underclass'!!
Someone who is part of a family who has not worked for 3 generations and has no role models for a work ethic is part of the 'educational underclass' unless they themselves have seen the value of education and strive to use it to break the cycle. Even then they will find it hard if they don't have paper and pencils at home, their parents won't find them a quiet spot for homework, they won't take them to the library, won't go to parent's evenings etc.
It is unfair. The rich are not going to be part of it. They may be useless in the parenting skills but they can farm them out with nannies, schools etc. they will still be disadvantaged beside their peers with parents with good parenting skills, but they have been caught early enough. Those without money need early intervention.
It is attitude, even if very poor you can use education to break out of a cycle. You can talk to your child, visit the library, draw, take them to toddler groups, socialise them etc. You are then not part of the 'educational underclass' because you are making sure that they can access the curriculum.
Sure Start has been mentioned, they are specifically there to help. Catch them early, if you wait until they are 7or 8 yrs it is more difficult and even more difficult when teenagers.