A relative of mine has some high student loan debt. I've decide to help pay down some of it. I called AES and told them that I want to apply the payment to the highest interest loan and for principal reduction only. They told me they cannot apply it only to principal.
According to the answer of this question, it's common for federal loans to not allow paying down principal. I'm unsure whether her student loans are federal or not. (How would I check that?)
However, searching the web, I see this bank website claiming that you should apply any additional payments to principal first. Are they only talking about student loans that are not federal loans? Or is it possible to pay down principal only on a federal loan?