As I understand, the argument goes like this: kids are required by law to go to school until 12th grade. If you let some kids test out of math classes and take

As I understand, the argument goes like this: kids are required by law to go to school until 12th grade. If you let some kids test out of math classes and take higher level classes, then by the end of the required schooling (12th grade), those students will be smarter and look better to colleges. Smarter and college educated people tend to make more and therefore there's an advantage of kids who test out. Meanwhile, kids who don't test out look worse at the end of 12th grade. In theory, yes, we should let kids finish at the same time. But required school is over at the 12th grade mark, which is also around the time colleges are looking at you. Most kids won't do non-required schooling to get to the same level as the other kids.