One, I don’t buy the argument that advanced courses are inherently finite. Kids have to be taught, regardless, and more kids in advanced classes just means fewer in the other classes. In terms of education spending, salaries are the overwhelming expense. Advanced classes only cost more if you insist they be smaller, and they don’t need to be smaller–what kind of fucked up logic says the strongest students should get the most benefit of the smallest student/teacher ratio?
Gatekeeping is a real problem in advanced courses, and there are many ways, explicit and implicit, that it can happen. Some schools just flat out require a certain GPA or teacher recommendation. Others let anyone take the course, but don’t offer much support and let people drop (I cannot tell you how many teachers I’ve heard explain that they make their summer assignments or the first grading cycle difficult to “clear out the deadwood.”) Others require resources that some kids can’t commit–like being available for mandatory labs/tutoring with no transportation provided, buying books/lab supplies they can’t afford. The final method is to just assign a shit-ton of pointless homework that is only really feasible for kids with a full-time support network at home.
A lot of this is because for things like AP, teachers live and die by their pass %, and opening enrollment–truly opening it, which means committing to supporting and keeping the kids you take–is terrifying. Teachers are very prone to imposter syndrome. Even rock stars worry that it’s not really them, it’s that they have “good kids” and if they take a broader range, the whole thing will fall apart. Gather AP teachers together, and the bitching about how they get kids that “don’t belong” begins almost immediately. It’s everyone’s favorite boogieman. Add that to the fact that many people think a tough, exclusive class implies a brilliant instructor and basically every factor is pushing schools towards exclusivity.
For whatever it’s worth, it’s not at all uncommon to have a school where the AP classes are dominated by Asians, the “regular” classes by white kids, and the “on-level” classes by underrepresented minorities. Whatever the OP thinks, that’s by no means a crazy hypothetical.
College Board recognizes this problem. They have been pushing open enrollment for the last decade and certainly the situation has gotten better. However, they still regularly point out that there are some 300,000 kids with PSAT scores that indicate they are ready for AP courses but are not enrolled in any. Presumably some of them have made a choice, but I’ve no doubt that a lot of it is gatekeeping based on what someone thinks an “AP kid” looks like.
I feel pretty passionately that there are a great many students who are underserved because people feel like education has to have an exclusive element to make it valuable. But the solution isn’t to just change which group has access.