How can we deal with the parents who did not agree that their child is weak in studies... and they put all blame in teacher?

A teacher is not a person that has a knowledge and they transmit this knowledge to the other people ( it can be child or adult). A teacher is someone how is a well communicate person, a person that can understand the other, put him self in the place of other. A teacher has the ability and skills to understand the environment. Nothing cold be done by force or by convincing. Sometimes, when you try convince you lose the target. So, to respond, there is now a recipe, there is only a sharing, parents are also the main part in the teaching process. By making them participate (of course at a certain degree) in the process of teaching, teachers can reach their target and make them participate to create a circle when you can find all the information needed to help the pupil or the child or even the student to go forwards.  

To explain the area where the child is weak and explain how it can be addressed so the parent can understand and offer the parent to join the class and observe their child for themselves and make a joint decision.

Also emphasis how the child has developed so that the parent wont think their child is being targeted unnecessarily.