Yes, Linear Algebra. Because if you take AP Calc BC as a junior (not uncommon here), what would you take as a senior?
Oh, I don't know...statistics? Proofs in secondary school mathematics? Problem solving in secondary school mathematics? Perhaps these courses aren't sexy enough because they merely create a foundation and don't come with a sparkly sticker saying, "
Dear IVY LEAGUE admissions officer, I did college-level mathematics when I was only 16! 
" Or, my cynical side is saying, because the latter two are actually
very hard, and we can't have hard classes bursting our bubbles.
I did a web search and found that we have high schools offering multivariate calculus and differential equations in high school, too. Either everyone is highly gifted or maybe (just maybe) a lot of these kids aren't actually learning this stuff in a meaningful way (though they can probably do the homework problems), and acceleration-itis continues unabated in American schools.
All I know for sure is that a lot of university professors in mathematics, engineering, and science are saying that students who took calculus in high school are failing the pre-calc placement exams in colleges. Well, I also know that the secondary school math courses I've seen offered around here and read about don't exactly fill me with confidence.
