I hate to quote Hillary but have you ever heard the term, "It takes a village"? What schools need to be successful is a three party contract.
Party of the first part: A student who strives to be the best he/she can be. They want a better life for themselves. They can accomplish that in many ways. Going to college to obtain a degree or a learning a trade among those ways.
Now, some kids do that on their own. They want to be better than their parents and they understand that they must knuckle down in school, get good grades and stay out of trouble. They are the center point of the contract.
Teachers, are an important factor as well. They know all kids need structure, discipline and education. They prepare for the weakest in their class all too often and thereby don't always push those kids at the top or in the middle. However, they know the kids at the top have excellent backing at home and those in the middle do for the most part.
Finally, the parents must buy into the educating of their kids. They have to push a little harder, stress homework and testing. Sometimes they just don't care.
What is a constant in the above? The teacher. He/she teaches all of his/her students. They try to spend a little more time on the slower kids but ultimately their job is to pull the best they can out of each student. They are a constant factor and the fact that most kids graduate with some avenue to success proves they do their job.
The variable here is the student and his parents and their home life. Is education a priority? Do the parents oversee the actions of the students? Do the children understand their role?
When you or anyone can explain how the teachers can be blamed, let me know. My argument will stand against innuendo.
Now there are weak teachers but you tend do damn them all because of a few and because some kids just don't nor ever will get it.