I would expect that it wouldn't be that difficult to select kids (perhaps based on prior performance in math classes, or overall GPA) to opt them into the class. This would be more fine-grained than simply shunting all kids into an advanced class.
That's how it works now, pretty much everywhere. Your 7th grade teacher recommends you to take the 8th grade algebra class based on your performance (or, at least, their interpretation of your performance)
The question is, where do you draw the line that qualifies someone to take 8th grade algebra? Who do you let in when there is 1 seat left and 3 eligible students to fill that seat? How do you make sure that the grades you are using to determine who makes the cut are accurately measuring mathematical ability of the students and not biased in some other way?
I'm not advocating for either the California or the Dallas solution to this problem, but both those school districts have identified that letting the 7th grade teachers make this placement decisions is a problem.