It may be standard-- and yeah, we heard a lot of this tripe, too, when DD was in high school.
The bottom line is that this kind of "prerequisite" can generally be waived-- I know of three instances in which our school happily did so (aside from in my DD's case).
She took AP coursework without "prerequisite" coursework in the subject (a year of honors) in physics, mathematics, English, and (if I recall) US history.
I think that for kids who are beyond MG, this kind of prerequisite requirement is usually toxic.
