You aren't allowed to pay towards the principal unless the interest is paid, and that is normal. I suggest the two-check system and denote what is applied to the principal. Nothing can be applied to the principal until the interest is paid off, that is what I've encountered with Sallie Mae. Any payments while in school are better than none, as your interest can be compounded into the principal at the end of your time at school (it screwed me a little bit- I paid, but I should have paid a little more instead of hitting the noodle house!).

You can also have an extra payment applied to only 1 loan instead of the entire loan group (I have approx 15 loans within 3 loan groups). Pay down the interest and principal on the loan with the highest interest rate. Good luck!