Baltimore would be a counterexample. They spend $22k per student, with a student-teacher ratio of 15 to 1. This still results in remarkably poor performance, with only 8% of students proficient in math and 22% in reading.

Culture and genetics would be next obvious explanations.

Here, we have to go through 4 state exams just to get to university. The first when you’re 11, the second at 14, then two consecutive ones at 17 and 18. There’s a national curriculum that the exams will be about, although the schools are free to add to it. So however you feel about the school or the teacher, you have to master the subjects enough to go through. And that means paying attention in class, cram before it, or hoping you can cheat. We have our own problem too, but the consensus among all the people I know that have moved to the US is that classes are easy there. Not a bad thing per se (better explanation, better understanding instead of rote memorizing).

An instructional challenge at the Ivy end, is incoming students having had such clear teachers, that they lack the attitudes and skills for wrestling with information to extract understanding.