My own personal experience from three years of being an undergraduate math instructor:
Typically during the first week of class, around 10% (six or seven students) will approach me asking if they really need to take the class if they got N on the AP Calculus exam, where N is usually greater than 3 and they rarely distinguish between AB and BC.
I encourage them to stay, because they're usually misjudging the difficulty of the course from the first week or so of classes. I can't recall any of them doing well. There's a lot of confounding factors in this account, however....
When I was in high school, I noticed is that it was possible to score the top mark on an Advanced Placement (AP) Exam by answering a relatively small portion of the questions correctly.
During my junior year, I self-studied calculus, and took the AP Calculus AB exam. I was very surprised that I scored a 5 (the top mark), because at the time when I took the exam, I didn't know some very basic things that were on the syllabus.
The College Board gives the raw score to AP score conversions for the exams that have been most recently released. The percentages needed to get a 5 are as follows: