They might not be dischargeable in bankruptcy, but a recent article I came across mentioned that a huge percentage of outstanding loans are technically in default. Those are loans that are way behind in payments.
Until 2010, and still on the guaranteed debt outstanding since 2010, if someone defaulted the government would pay it off. This would not discharge the debt, it would just become a debt owed to the government.
http://www.nolo.com/legal-encyclopedia/what-is-federally-gua...