I imagine this kind of thing varies between countries. It appears the standard in the US is that if repaying the debt would be an "undue hardship" then a judge may discharge it in bankruptcy. You can also file a different kind of bankruptcy that can change your repayment terms, though I think mostly the schedule rather than the actual amounts.

This is all from ~5 minutes of research though, so take with salt as appropriate.

http://www.studentloanborrowerassistance.org/bankruptcy/