As bad as the academic results of gov't schools can be, that is not the main problem. Far worse is the leftist propagandizing that does go on.
For example, it you love liberty and free markets, you should recognize that it is far more damaging to propagandize the students to believe in statism (which is done constantly).
If you are a religious type, you should recognize that the constant promotion of the gay agenda is putting your kids on the highroad to hades.
If you believe in the importance of scientific rigor, then you should go ballistic over the politically-based lies of "climate change."
This could go on forever, but the point is: the propagandizing they do is far more destructive than the teaching they fail to do.