I don't think such schools exist today, but when they did, it was for those who got expelled from regular school for things like not saluting the flag or other problems that the Society created itself. As for the quality of the "education", it is likely to be wretched because there are so many things that simple logic would be able to refute what the Watchtower Society says as doctrine or to support their doctrines. After all, they say that reading four years of the Asleep! magazines is equivalent to a college education. The University of Stupidity!
Home schooling is in fact fairly common. I know of one family where the kids didn't want the holidays and the worldly association that comes with regular school. The parents chose the textbooks, rejecting books on the grounds of a single "dirty" word (and they called the word "crap" dirty). And they were biased for the Watchtower doctrines, like creation (I feel evolution, while not absolutely proven, is much more plausible than creation anyway). But it's those "bad associations" that sucker most of them into home schooling. Usually the quality of such "education" is crap and they miss out on all the holidays and field trips (unless you call a trip to the A$$embly Hell or to Beth Hell a field trip).
How much they miss out on! I remember field trips to the Boston Museum of Science. I have been on a math competition team, which was a lot of fun. I have also seen field trips to animal farms, to the zoo, and once to a maple syrup processing farm. These trips help integrate what you learn in the classroom with real life, and the Witlesses miss out on them. Not to mention college prefrosh trips, trips to computer fairs, and the like. If the parents would provide those in their home schooling (and make sure that the education material isn't all pro-JW), it might be OK. But most of them miss out on so much education from this.