A) Interest represents risk as well as the time-value of money
B) A non-dischargeable loan is risk free to the lender.
Given those, there should be two products:
1) Offering a loan dischargeable in bankruptcy, at whatever rate the market will bear.
2) Offering a non-dischargeable loan, with interest capped at the current rate for a 10-Year Treasury Note.
If there's no risk, a bank doesn't get to add it in, and the money is cheap enough that the student can pay it back.
If there's risk, a bank gets to charge whatever they can, and a student has recourse to bankruptcy.