Teachers, today are required to deal with much more than education. In some sense, they are raising children and are expected to do this by administration and parents. Whether it be that the parents are working multiple or demanding jobs, or they have too many children to handle (the older ones neglected while the babies take up the parent's time), or the parents are not qualified or mature enough to teach them, it is the teacher who must take up the slack. In some cases, they interact with the child much more than their parents.
If a teacher has a packed classroom full of children with behavior problems, learning disabilities, attention disorders, far behind grade level, ESL students, students struggling with terrible home lives, how on earth can we fault them for not getting stellar grades on tests? I have taught classes with all of those issues and sometimes getting through the day is a challenge.
We as a society are holding teachers responsible for children in every aspect while becoming more unrealistic about parenting as well. Parents are not supported or even educated on what is truly important in raising a child (hint: it's not buying them everything their heart's desire). They are taught that it is the teacher's fault if their children are not learning. And what has been broken is the supportive, symbiotic relationship that MUST exist between parents and teachers for a child to be a successful student.