Oh, that’s rather different, IMHO. Yes, I agree that the rigor at most BS is far more than standard public school. Whether it’s equal to an AP course is something else (again IMO). I totally misunderstood your post at first.
I think it’s common for many BS to have most kids taking the same level courses in the first couple of years and then spreading their wings. Some allow kids to take advanced programs in their first year but I think it’s mainly in STEM ( and languages where you are testing in so level mainly depends on preparation).
And yes, some kids take the AP some don’t and some are required to take it.