I am a teacher and the Principal wants me to change a grade for a particular student. At least, not if you can make a reasonable case for the grade. Record-keeping is very important. It always really helps if you were clear to students beforehand as to course expectations. If you gave a student a failing grade, some notice must have been given to the parent.
Parents can challenge the contents of student records.
Parents have the right to inspect the materials that teachers use in their teaching according to The Education Empowerment Act of 1998. But the law specifically states that despite the input from parents regarding these materials, these powers do not restrict teachers in the assignment of homework. Parental involvement in public education is public policy. Therefore, it is wise to examine the policy in your school district regarding homework. Education Code 44034 would allow such things if it is intended to improve your teaching .