We don't have AP courses here that I know but in general it seems fair that anybody who wants to should be allowed to take a course specified as a prerequisite but only those who pass the requisite course with the required grade get into the AP course. In our system you go from high school to university. Anyone who qualifies for university can do first year in all (that i know of) but you have to do REALLY well in first year medicine, law, vet science etc to be selected to Continue. There are some reserved spaces in some of those for specified groups but there is still competition for those.
of course advanced courses are most beneficial to high achievers not PG but it is hard to see how they would benefit someone who wasn't at least high average.
The girl who couldn't keep up with history may have been memorising instead of processing because she was out of her depth.
