Partially correct on your second point, parents are, from time to time, the worst about seeing inadequacies with their own kids and much quicker to point out others' shortcomings.

Parents need to come to terms with the fact that if their child isn't doing well in class that maybe it isn't the teacher's fault, that it could possibly be that their child doesn't understand something about the subject that is preventing them from learning something new.  And that the teacher, in a class of 20-40 kids doesn't have time to get to every child for 1-on-1 help.  Parents should be involved in their child's education to help if they can (don't get me started on common core math >.< ).  And if at the end of the year their child doesn't know what they need to, they have to face the facts that your child should be held back until they learn what they need to, if they do not learn it and move on it becomes a compounding effect.