Such pessimism...600k would be fewer than 1 student in 10 in California. Even in California 1 family in 11 or 12 probably still cares enough to take its children out IF the family knows what has happened.
The current problem in California is that there has been a news blackout on SB 777. Either it isn’t reported on or, when it does come up, people are told that the legislation really doesn’t change anything and that the concerns expressed are much ado about nothing. Consequently, the key will be whether the various organizations involved can get the truth to the grassroots. Various initiatives are under way in that regard in CA, and I think that it is just a matter of time.
FYI, there is already a group of Russian baptist churches in the Sacramento area that is discussing taking all of its children out of the government schools. That would be at least between 3K and 6k children. Based on what I know I would say that the exodus of these children is highly probable.
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.