Another qualification for Public Service Loan Forgiveness (assuming this is the program you're referring to) is that you must be working full-time while you are making the payments - which they define as "if you meet your employer's definition of full-time work or at least 30 hours per week, whichever is greater". Most assistantships I'm aware of are offered based on 20 hours of work per week, which would not meet that qualification (unfortunately). ☹️