Why are parents the key to the overall success of their children and their children's education?
So much of educational success begins at home. Whether or not a child gets enough sleep and eats properly is the parents' responsibility. Setting high (but reasonable) achievement expectations is also the parents' responsibility (though of course teachers should support them), and supervising out-of-school work needs to be done by the parents in the home. All of these things--and many more--are the foundation for what teachers are trying to do in the classroom. Of course, students do make choices which influence their educational outcomes, but when the school reinforces the parent, the result is generally a productive, educated young person. When either component is lacking, the outcome is not as likely to be successful.
For special needs children, parents are the most important factor in how successful their children's educational experience will be. Parents must not only do the things mentioned above, but they must also be much more vigilant in supervising what is happening with their children in the classroom. This is not because special education teachers are bad but because resources are scarce and sometimes essential needs are not being or cannot be met. While this can also happen with "regular" students, they are at least able to help identify the needs; special needs kids are not always able to do so and need their parents to be their educational advocates.
At some point, what a student chooses to do (or not do) in the classroom is more important than anything else; however, parents who are involved in their children's educations and partner with their local schools are giving their children the best chance for educational success.