I don't know if it's as simple as it being always being either causation or correlation. Education level is used as a proxy of potential wellbeing indicators.
Yes, there are rare people who have never finished high school and had material success, probably most drug dealers who cashed out and bought franchises or real estate. So logically then general causation is ruled out.
But for most people without high school degrees it's extremely difficult to disassociate their lack of education with later material success.
50 years ago it wasn't that weird to find a person with a good combination of smarts and work ethic who just happened to not graduate from high school, for whatever reason. The lack of a HS diploma isn't necessarily a stain.
In the year 2015, graduation from high school is a much easier in almost all ways. If you can't manage to graduate there is something seriously wrong and I probably don't want to hire you.
This is why I'm always terrified of trying to push more and more people through educational hoops: the ones who don't make it are that much more weird and excluded.
Yes, there are rare people who have never finished high school and had material success, probably most drug dealers who cashed out and bought franchises or real estate. So logically then general causation is ruled out.
But for most people without high school degrees it's extremely difficult to disassociate their lack of education with later material success.