What I don't understand if people's schools are so bad why send them there.
Some people simply don't have a choice. They are unable to, for whatever reason (financial, etc), keep their kids at home to homeschool.
As for the original post, I would have left the teacher out of the equation and simply said to my child that teachers are like anyone - sometimes they are wrong. It's that simple. In our house, I'm in charge of our kids' education. Period. The school has made recommendations that I have followed, and others that I have declined. I have the final say.
I have an issue with school staff assessing students and implementing education strategies based on their limited setting-based and individualized exposure to each child. They simply don't know my kids the way that I do. They don't know the "whole" child.
I also have issues with curriculum gaps (lack of "drill and kill" for math facts being one - some kids, like my ADHD son, simply need it). If I left the school in charge of math, my son would reach high school completely calculator dependent.
Might I also mention, while we're on the subject of math, that in my grade 6 daughter's class, they are required to use calculators rather than having been taught things like basic percent calculations. I could go on, and on...
...and on, and on... lol
Not to mention the fact that each teacher has my child for one year (usually). After those ten months are through, the teacher moves on to the next batch of kids and what happens to my child is no longer relevant to her. Meanwhile, I have the responsibility of ensuring that my child is ready for adulthood, so I see the impact of poor decisions later on.,/b>
Who's responsible? Me. Period. 
