I have an apology to make. Some years ago, when I was a student, I worked as a mentor and teacher for the Aimhigher programme, which was designed to encourage gifted pupils from disadvantaged backgrounds to apply to university. In return for room, board and sandwich money I spent my days giving lectures about exciting books, and my nights trying to find where my eager pupils had stashed their vodka.

The most important part of the job was persuading them that higher education was worth it. It was talking them through their worries about debt, and telling them that it would be OK. It was telling them to go ahead and study what they loved, because university was a route to employment, and they could get a great education without having to worry that they'd end up poor and sick because of it.

I want to apologise to those young people, because I lied to them. I didn't mean to lie. At the time I entirely believed what I was telling them: that taking on education debt would always be worth their while; that they should go to university for the love of learning, because they were talented and brilliant and longed to study – and not just with one eye on the well-paid jobs that they would surely get after graduating. Wherever they are, I'd like to say sorry for trying to persuade those kids that higher education was meant to be about reading lots, meeting new people, and getting excited about ideas.

Because it turned out that it's actually just about making yourself more "employable". It's about fashioning yourself into a walking CV to compete for a stagnant pool of graduate jobs that are paid less in real terms every year, and taking on a rotten amount of debt in the process.

This week news quietly broke that the extra money a degree is supposed to earn you over the course of a lifetime – the "graduate premium" – is going down year on year, even as tuition fees are going up. Today's students are paying nine times what I paid in 2007 and, according to one government adviser, up to 40% of the class of 2012 may default on their student loans.

The Department for Business, Innovation and Skills – named in honour of this government's order of priorities – is keen for us all to remember that graduates can still expect to earn an extra £100,000 in a lifetime. However, if you want to play the game of obfuscatory averages, that figure must be balanced against the average debt of £44,000 that students are taking on. Over a working life of 45 years (assuming they can afford to retire) that would leave students with net earnings of just £1,244 a year. That's if they can actually find a job that pays enough to live in the town or city where the job is located. These are the sort of mercenary calculations to which the value of learning has been reduced.

The graduate premium was always a silly idea. GCSE maths is a requirement for most college courses, and anyone smart enough to pass it can work out that just because a higher degree was a ticket to a great job in 1967, when the economy was booming, doesn't mean that in 2014 – when university attendance has increased tenfold and youth unemployment stands at 22% – it will lead to a good job, or any job. And yet ministers still insist that students should shoulder lifelong debts to receive the same education they themselves enjoyed for free.

The government speaks the language of aspiration, but what it is really counting on is desperation. The gamble taken in 2010, when the Liberal Democrats went back on their promise not to raise tuition fees, depended on those fees replacing the enormous cut to the block grant to universities that happened at the same time. Students are now taking on debts to fund this government's cuts to services that were state-funded for generations for the public good.

Official estimates suggest the number of student loans that will ever be repaid has been dramatically revised down – meaning universities are facing what the same government adviser called a "fiscal time bomb". So ministers know full well that today's students have less chance of earning even enough to repay their debts – but if they stop taking on those debts, the bottom will drop out of the British university system.

For generations social mobility and higher education were synonymous in Britain, and much of Europe; but those certainties have been shattered in less than five years. If I could go back in time and speak to the pupils I mentored in 2006, I'd still tell them to go out and get the best education they can – not because it'll get them a good job, but because reading, learning and expanding your horizons is necessary if you're going to understand what's being done to the world around you, and change your collective circumstances. Unfortunately, Aimhigher was closed three years ago to cut costs. So much for aspiration.