If that were true, you'd expect competitive pressure to push the interest rates down to 1% or lower. Do you claim that the banks are colluding (think "The Informant") to rip students off?
I should have clarified: zero default risk.
There's still the time / opportunity cost of money, and it's possible that a loan might not be repaid at any given rate. The interest would reflect this. And inflation risk would have to be factored in.
There's also a case that might be made that offering of loans isn't entirely competitive. Dynamics of that, and implications on interest rates resulting from that condition could be interesting to explore.
Also, risk of default is non-zero. Google the Brunner test.