This one may be hard to accept, but you shouldn't assume that because your child makes straight A's that he/she is getting a good education. The truth is, a lot of times it's the bad teachers who give the easiest grades, because they know by giving good grades everyone will leave them alone. Parents will say, "My child has a great teacher! He made all A's this year!"
Wow. Come on now. In all honesty, it's usually the best teachers who are giving the lowest grades, because they are raising expectations. Yet, when your children receive low scores you want to complain and head to the principal's office.
Please, take a step back and get a good look at the landscape. Before you challenge those low grades you feel the teacher has "given" your child, you might need to realize your child "earned" those grades and that the teacher you are complaining about is actually the one that is providing the best education.
When I was in college (once I got serious), I would seek out the teachers who had the reputation of being the hardest; I knew I would learn!