That's a very large percentage to leave the "free" government schools over this particular issue. As I mentioned above, this is far from the first occurrence that would make parents consider alternative education. I think that the majority of those who are going to choose private education have already done so.
You may be right about some organized groups, such as the Russian Baptists, who will leave government education over this point. Since Slavic Evangelicals (Ukrainians and others, as well as Russians) often have large families, this could come to tens of thousands throughout the state. However, I believe the 600,000 figure is excessive.
Most parents have no idea what is really going on in government schools (this is true in other states, too). A great deal of money is spent in California and elsewhere by districts to perpetuate the myth that their schools are somehow “different” from the schools parents are reading about or may have heard about. In the case of SB 777, it is now clear that no school can be “different.” Once parents and churches are aware of that, a larger percentage than 8% or 9% may leave. One key is making churches aware that they can provide a very good alternative to public schools at a very low cost without starting a traditional Christian school. The lower price point that is possible today will make it possible for many more children to leave.
The school establishment in CA is very nervous about this.