Private schools, even ones that receive less tuition per student than public schools of the same district prove this point because they can set a higher bar and more readily kick out disruptive students.
Nationally, per student cost at Catholic parochial schools is roughly 1/2 of public schools. (Couldn't find California specific numbers, but it tracks what I've seen.) That's cost/spending; tuition is even less. Special education programs eat up 1/4-1/3 of public school budgets, so there's still a gap after accounting for that.
1973 was over 50 years ago. Even the poorest, most "backward" states have long ago largely remediated spending inequities. What you hear today to support claims of inequity are cherry-picked esoteric cases to drive outrage, just like every other topic today. In reality there's clearly much more going on than what advocates for this or that would ever admit.
In any event, "picking your customers" is exactly the point of the previous poster(s). Money doesn't really explain what we're seeing. Once upon a time there was a very strong correlation, and people presumed causation, but now it's quite clear that the causation wasn't there. Certainly not directly, which is why throwing more money at the schools isn't helping.
Public schools have to take children that do not want to be there and make it everyone's problem, and their classmates suffer for it. Maybe this public school policy is the issue that should concern you.