I had calculus in HS, it was pretty basic stuff, I could diff and int some things. I hit college and took calculus, again, Honors level, and we covered everything I learned in HS in Day one of the first quarter and Day two of the second. The rest was ... obscure. The prof wrote on the board with chalk with one hand and erased with the other. Some folks in the class didin't bother taking notes, they just watched. I vaguely recall something called Green's Theorem, but not what it was about of course.
The text book was zero help, no overlap with what the professor was saying. I got through it somehow.
My first semester in grad school we all took a course called "Bonding", same kind of thing, book was zero help, the professors wrote, they didn't erase at least, but I really had no clue what was going on. None. The first test came and I made something like a 57 only to learn it was second highest grade in the class. Your first semester you don't really know anyone yet and everyone looked like they understood it all, but nobody did. So, I somehow slid through with a "P". (They had an odd, to me, HPLF grading system, if you got an F you were out, if you got two Ls you were out (Low Pass), if you got all Ps you were fine, and they just didn't give any H (High Pass) grades. Fortunately, we had about a year and a half of classes and then none from there on. That meant our tuition dropped to zero which was fine.
There was another guy from UGA who came in a year after me, I knew him, he was wicked smart, but somehow unfocused, and he ended up disappearing one day. It was guys and gals like me who muddled through mostly.