It is one hundred percent legal for a teacher to give the grade he desires to give a student, although usually there needs to be a basis for such reason. Police are not involved in this. But; the administration - like principals, superintendents, department heads - are also people too, with the community watching about dropouts, and if it can be convinced that an unfair grade was given, they, you, and the teacher may have a sit-down to discuss a compromise, provided evidence of wrongful grade is there for them to see, and the student is professional about it - and really cares (not just because his or her parent's care).
But of course, you could retrieve the assignments, exams etc., and then contact the scholar boards or administrations to try and dispute the grade. If you can convince the boards, you may not be able to solve the problem, but the teacher would get in extremely bad situation due to the school district and possibly the state. In the end, the teacher would have to be forced to change the grade because after all, the matter wouldn't be a worth of the job.
The law doesn't have anything to do with this. You need to pressure mentally. Scholar boards won't do anything except threaten a bit, the teacher himself has the button and the finger to press it.