I went to a few schools when I grew up, some of which did not have good academic reputations, and they just let me take the class (and exams) a year higher for subjects I was good at. Admittedly the time-table confusion for one year did mean for some classes I was sitting in a desk in the corridor, but at least I didn't have to resort to time travel like Hermione did in the harry potter books.
Being good at maths is quite common I would have though (compared to e.g. being good at History) but perhaps not as common as being good at a language (if you are bilingual). Do Hispanic kids get put in the same year in Spanish classes as beginners or do they get a more advanced course?
"Probably the best operating system in the world is the [operating system] made for the PDP-11 by Bell Laboratories." - Ted Nelson, October 1977