Happy graduation! This year's graduating class is the most indebted in history, with $56 billion in student loans to pay back.

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That's according to an analysis of an Education Department study by financial aid expert Mark Kantrowitz, first reported by Mic's Zeeshan Aleem. Kantrowitz estimates that 71 percent of this year's graduating class took out loans, and that on average, students with loans owe more than $35,000:

Kantrowitz's estimates of the total amount owed are slightly higher than the other main source of information on student debt, the annual reports from the Institute for College Access and Success. But both data sources agree that student debt has risen considerably. In 1996, 58 percent of students borrowed to attend college; adjusted for inflation, they owed less than $20,000 at graduation.

One reason student debt has increased so sharply is that the burden of paying for an education at public colleges and universities, which the majority of college students attend, has shifted from taxpayers to individual students. States paid more than two-thirds of the cost of higher education per student in 1996; now they pay only about half. Those cutbacks have led to tuition increases. And while state spending is now rising again, it's unlikely to change the overall trend. The class of 2015 will only be the most indebted in history until the class of 2016 graduates next year.