You mean college?
I once interviewed for a phd position where I was asked why I wanted to go to grad school. When I said it was because I wanted to teach people who were really interested in the subject and that I thought the only place that was possible was at the university, one professor couldn't help informing me how naive I was wrt random students. I later confirmed this when I was TA'ing undergraduate physics majors. Only about 30% were actually interested in those voluntary and self-selected modular classes that they paid [their own] money for. The other 70% were there because they needed "a degree" in order to get a job. So I guess, technically, it was only voluntary in spirit, not in application.
Of course, 30% is a lot better than the 5% found in high school (which is also voluntary, but free, and only somewhat modular).
Point being, it's impossible to create a system in a society that uses education for other reasons than learning (e.g. selling credentials or regulating the influx to the labor markets). In other words, "nature" finds a way.