I have absolutely zero personal experience with APUSH, but one of the faculty in our school has written items for the test, and I will say this individual runs a pretty rigorous AP class. We also happen to have two history faculty with content area doctorates (i.e., not administrative EdDs), including this faculty member (who taught at the university level prior to taking this position).

I'm going to guess that instruction is highly variable, depending on the school. But in our building, which is not even a comprehensive, hence far from being an exceptionally high-achieving high school, it can be pretty respectable.
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...pronounced like the long vowel and first letter of the alphabet...