Parents shouldn't get all the blame for their child's poor performance in school. Everyone has to take the blame because it takes a village to raise a child, so everyone should take the blame even the child. Students might not be used to the teacher's teaching method or the student can be smart but very lazy so chooses not to do any work. This is what hurts most students in school and that's their attitude towards school work.
Most parents are as concerned as they should be with their child's performance in school so that can cause the child to slack off in school because the the parents attitude. Then parents want to know why their kids are failing in school on their report card. If you were concerned and involved from the beginning then the report card wouldn't be a surprise to the you (the parent).
On the other hand it can also be the child's fault. The child has the most control over what grades he or she makes. As long as the child focuses in school and does what is supposed to be done I don't see any reason the child should fail. So everybody has to work together in order for the child to do well, compromise and getting good grades should be that much easier. Therefore, the parent should not get all the blame because the student,teacher, and parent should work together to make things work.