I don't think they are as much worried about the quality of instruction as they are afraid of the competition this affords parents in their choice of educating their children. Whether in public school or home school a student can have poor instruction and school officials need to clean up their own house before they worry about somebody else's. They are just flat running scared.
Be scared. You can't help that. But don't be afraid. Ain't nothing in the woods going to hurt you unless you corner it, or it smells that you are afraid. William Faulkner












Another problem comes when a kid shows up in the middle of the school year. They must take the kid, but no funding has been provided for them. That is the fault of the legislature and how schools are funded, but it does have a tendency to pit the school system against the homeschooler. Many of these kids are the homeschool 'failures' rather than the successes, so that gives a clouded view as well.
There really are a fair number of folks currently homeschooling who are not qualified. Child neglect is often involved. IMHO, a functionally illiterate adult has no business homeschooling their child either. With no oversight at all, I think more of this falls through the cracks. Some states have no checks/balances at all and I think that is a disservice as we all know (even just from articles you post on your blog) that not everybody treats their children as precious. Unfortunately, even if they have checks/balances, no funding is included for them to be implemented--again, pitting school system against parent. ok...off my soapbox.
In addition, I think the key to the argument is what the parents cited are teaching. Religion in this case frequently means a fundamentalist brand that eliminates evolution as a factor in science and twists some of the other aspects of today into rather unbelievable forms.
And as Stephanie suggests, child abuse may be part of it. And the home schooled may do well on the tests because fewer of them are involved on going on to colleges or universities. The tests aren't required if you aren't going on. People who want their kids to go to college will do better, one way or another, than those who don't. That's a generality, but I expect it will hold up whether the child is home schooled or is in public school.
I know some home school families and no child abuse/neglect is involved and I take some offense at this chrge being leveled. It is a recent argument raised against home schooling by those who don't belive in it or want all kids to be brainwashed in the public schools and is no more true than any of the other fallacious charges that have been raised against home schooling. I am sure there are some people out there who abuse thier kids that are home schooling but that is not what all home schoolers do. Just like there might be a few illeterate people who homeschool there kids out there but that is not again all of the homeschool kids. Sterotyping is dangerous.
Although I no longer tutor homeschoolers, I keep in contact with friends who still do. It's a relatively friendly atmosphere in this state, depending on how you define friendly (there is some regulation). However, I have to tell you, I've met some people at the state homeschooling fairs that make me glad that they aren't my parent!