Its wrong to compare white and non white kids by class. A lot of non white kids have parents with similar incomes as white working class parents but a lot of them have middle class aspirations or were middle class in their native countries but had to do low skilled jobs because their qualifications are not recognised in the UK. They get classified as working class but were or really want their kids to be middle class hence more pushy with their kids at school
White working class people are working class because they are not academic and relied on gaining manual skills to make a living. They don't value academic achievement because they did not need it.
I think its unhelpful to classify people as underachieving because they are not doing well academically but ignoring the fact they don't want to be academic in the first place because they don't think they need to be.