Two points about this.
One, why tax graduates? In other EU countries, free is free as in beer and you never need to pay anyone anything, for higher education.
Two, why double-tax the future workforce? I mean, they'll pay taxes once they start working anyway. And obviously, those who do go into higher education will end up making more money, and therefore pay more taxes. The state will get back its penny'orth alright, so what's the point of adding a cherry on top?
Full disclosure- I'm from Greece originally but graduated from a UK university, and am repaying a tuition free loan, to which I was entitled as an EU citizen; so personally I benefit from the system (because I'd probably not be eligible for free education anyway). It still doesn't make sense to me.