While I'm in favor of free/cheap education, this made me think of a convincing argument for the student loan / US system. Those who benefit from the education are "taxed" for it in the form of loan payments for a few years, those who don't benefit (directly) from the education are not.
I guess ideally you could balance the benefit to society vs the benefit to the individual getting the education and subsidize it proportionally. But that's obviously not practical so we should probably push closer to the society end of the margin of error.