If the parents that do this to their kids use the "schools aren't safe", "schools aren't teaching what I think they should be", or "schools are failing our children" excuses should *get involved* in their local school, and encourage all other parents to do the same. If their schools really are falling behind in some way, it's *THE PARENTS FAULT* for not being involved.
I don't buy it. I went through public school in one of the wealthiest counties in the US, and saw the problems first-hand. If I have a problem with the school system, it's way easier for me to pull my kid out and educate him/her at home, or at a private school, than leave him/her in the school that isn't properly educating. It is not my responsibility as a parent to fix the schools. My responsibility is to my children. If the school isn't providing the level of education I deem my children need, I'll go elsewhere and they can keep catering to the lowest common denominator. If you follow good curriculum, you can get at least the same level of education at home as you can in public school.