My older sister has a PhD and she said in grad school she was classified as an employee but not a full-time employee. To qualify for PLSF, in which she is currently enrolled, you have to be a full-time employee. Another consideration, as mentioned above, is your classification as a student. In order to have qualifying months/time toward PSLF, you also have to be making full (based on your repayment plan) payments. Most people would defer those payments while in school. If you have other questions, I can definitely ask her based on her experience.