The text of the article further suggests that many of the students who scored well on qualifying exams weren't taking the classes because they didn't have the parental involvement or default assumption that they would take it.

(Note that the opt-out group was selected from the general population based on the results of an exam; it wasn't a population-wide opt out. I suspect from your comment you didn't see that?)

That’s all.

> Within two years of this policy, 94% of eligible students are taking these classes.

Preselection.

Additionally, about half of the entire student body took the advanced class.

Considering the fact that the advanced class is Algebra I, I’d wager that just being on the right half of the bell curve would be sufficient to pass. Half of the student population = the right half of the bell curve.

Additionally, I didn’t see whether they were doing individual tutoring, which would have an impact on the results.

Seriously tho I think it will. I’m sure a foundation is important in many fields but one can get pretty far by jumping in. I remember my high school physics teacher take us on a wild ride of quantum mechanics for three weeks. If nothing it opened up a lot of future reading