you don't need a diploma, you need a work ethic

Tyler Cowen of Marginal Revolution on education and signaling, here

If education is pure signaling, just give everyone a standardized test in seventh grade and then close up the schools. But the process of self-image formation, at least for most people, is far from complete at that point.

That being said, education will look like what the signaling model predicts. It will be about subtle brainwashing, image, and learning markers of status. What the signaling model misses is how important those features are for your subsequent productivity.

Nerds will hate education and tend to embrace the signaling model. Their sense of self is often formed quite early, and they do not why so much time should be wasted in school. This is one reason why the signaling model is so popular in economics.

and here

We find that employer learning about productivity occurs fairly quickly after labor market entry, implying that the signaling effects of schooling are small.

Here is much more. And here is more yet; this second paper estimates the speed of employer learning and uses that estimate to bound the value of the signal at no more than 28 percent of the value of education. I consider this devastating to the signaling hypothesis. How can ?? years of schooling be needed to signal your quality, if your employer often knows your quality within months?