Professor marked absent even though I was present for internals

one of my professor has given me zero in internals. she marked zero on the ground that i was absent on the days when viva and test was conducted. I was present on both of the days and i requested her many times and even provided many witnesses that i was present. now i have wrote two letter to the principal but there is no progress. she still do not consider the 6 witnesses(students of my class)and stick to her records. I am feeling harassed being calling her and writing letters to principal. under which law i can file a case and juvenile act also applies to college students? any other advice?