Rather than link you to an individual study, here is a link to an NYT Magazine article that summarizes many studies about how the attitudes of high-SES ("rich kids") students are more highly valued ("better") than those of middle class and poor kids. You can find all of the primary source studies via the article.

"In her book “Unequal Childhoods,” published in 2003, Lareau described the costs and benefits of each approach and concluded that the natural-growth method had many advantages. Concerted cultivation, she wrote, “places intense labor demands on busy parents. ... Middle-class children argue with their parents, complain about their parents’ incompetence and disparage parents’ decisions.” Working-class and poor children, by contrast, “learn how to be members of informal peer groups. They learn how to manage their own time. They learn how to strategize.” But outside the family unit, Lareau wrote, the advantages of “natural growth” disappear. In public life, the qualities that middle-class children develop are consistently valued over the ones that poor and working-class children develop. Middle-class children become used to adults taking their concerns seriously, and so they grow up with a sense of entitlement, which gives them a confidence, in the classroom and elsewhere, that less-wealthy children lack. The cultural differences translate into a distinct advantage for middle-class children in school, on standardized achievement tests and, later in life, in the workplace.

...The disadvantages that poverty imposes on children aren’t primarily about material goods. True, every poor child would benefit from having more books in his home and more nutritious food to eat (and money certainly makes it easier to carry out a program of concerted cultivation). But the real advantages that middle-class children gain come from more elusive processes: the language that their parents use, the attitudes toward life that they convey. However you measure child-rearing, middle-class parents tend to do it differently than poor parents — and the path they follow in turn tends to give their children an array of advantages. As Lareau points out, kids from poor families might be nicer, they might be happier, they might be more polite — but in countless ways, the manner in which they are raised puts them at a disadvantage in the measures that count in contemporary American society.

What would it take to overcome these disadvantages? Does poverty itself need to be eradicated, or can its effects on children somehow be counteracted? Can the culture of child-rearing be changed in poor neighborhoods, and if so, is that a project that government or community organizations have the ability, or the right, to take on? Is it enough simply to educate poor children in the same way that middle-class children are educated? And can any school, on its own, really provide an education to poor minority students that would allow them to achieve the same results as middle-class students?"

http://www.nytimes.com/2006/11/26/magazine/26tough.html

As for work ethic, how are you defining work? Time on task? Emotional labor? Social loafing? Intrinsic motivation? Extrinsic motivation? Protestant Work Ethic? Locus of control? Prosocial behaviors?

There are at least a dozen different academic disciplines that study issues related to work ethic, so the question of how work ethic correlates with SES isn't really answerable as it currently stands.