CONTINUOUSLY billed incorrectly from Navient

I am continuously, and I mean CONTINUOUSLY billed incorrectly from Navient. The keep billing me as if I am graduated from school, and I am not. My contract with Sallie Mae states that I have 6 months after graduation to begin making monthly payments, that include interest and principal. For now, while in school, I make monthly interest payments. EVERY semester I have to have my school send these incompetent people proof that I am currently in school, and also, my date of graduation is given to these people!! But within a few weeks, they are billing me for full payment AGAIN! Then I have to make phone calls, contact my school, etc, etc. I do not have time for this, nor do I feel that this kind of continual harassment from Sallie Mae is right! Please help me do something to resolve this matter with Sallie Mae. I will forever be in your debt. Sincerely, E. G.