I've heard numerous times that less educated people have lower work ethics, but I've never seen any scientific papers on the subject. Is there any correlation between work ethic and level of education?
A study by Hill and Rojewski suggests that having poor work ethic is related to not completing school. As I interpret this, this would induce the correlation you are asking about.
Hill and Rojewski did a study with the goal of determining if work ethic differed for 152 9th grade students categorized by risk of school failure or dropout.
The motivation was the idea that "[youth at risk of failure or dropping out] exhibit poor attendance habits, lack of interest, and have discipline problems, and in the community they are often at odds with the law. These behaviors limit opportunities for success, either at school or at work, and their existence prevents people from fulfIlling their potential throughout their lives."
Why does this relate to your question?
Hill and Rojewski wanted to check if work ethic was indeed predictive of being at risk of failing. This question gets to what you're asking about because if people with poor work ethic fail or drop out of school, they will have lower levels of education.
To measure work ethic, students completed an Occupational Work Ethic Inventory.
To measure risk of school failure or dropout, "respondents were asked to indicate whether during the past school year they had been disciplined by a teacher (detention or extra study halls), suspended from school, had cut classes, been in trouble with the law, thought about dropping out of school, were disinterested in school, or disliked doing schoolwork that required some effort."
(The validity of the above measure of risk of school failure is supported by an earlier study by (Bryk and Thurn, 1989).)
The data indicated that being dependable was the key issue underlying differences in work ethic of students categorized by varying levels of at-risk behaviors. OWEI mean scores for at-risk students were lower than those for students who were moderately at risk, and mean scores for students not at risk were higher than scores for all others.
This study found that in a non-random sample of 152 9th graders, the questionnaire-based measure of work ethic correlated negatively with the self-reported measure of risk of school failure or dropout.
Bryk, Anthony S., and Yeow Meng Thum. "The effects of high school organization on dropping out: An exploratory investigation." American Educational Research Journal 26, no. 3 (1989): 353-383.
Hill, Roger B., and Jay W. Rojewski. "Double jeopardy: Work ethic differences in youth at risk of school failure." The Career Development Quarterly 47, no. 3 (1999): 267-279.