I did take a first-year course in Astronomy, but it was so easy I got an A+ without even really studying (the course material included things that I'd helped my little brother learn for his Year 9 SATs and the final exam was a 15-minute multiple choice paper!).
Yeah, I imagine some schools are like that, but the level of challenge is going to vary greatly based on the competitiveness and philosophy of the school. Just like most things, one or two examples doesn't mean that is a general rule. Take for instance my uni. You'd not have taken a first year test because there weren't tests. I took exactly one test, and that was given by an ecology profession guest teaching from another uni. Most of our grades were based on papers, and we wrote a f*** lot of them, but some was on class contribution and other work. And I doubt all the freshmen suicides at Cornell are based on failing work that you could have passed with a minimum of effort.
I often don't think of my experience as a typically American college (and, yes, there is a reason we call our universities "college"). We didn't have lecture halls. We didn't have more than 15 people in a class. But there are plenty of colleges in the US which are different than the norm, and even traditional universities which would challenge even those who've sat their A levels. Most of the time the more competitive schools are more expensive, although some public universities are highly competitive. Being really excellent at one thing or another doesn't mean a school is going to be challenging overall. Like anything, research is going to be key.
And most public (in the American sense) high schools are not going to have much astronomy in their high school science classes. Maybe a bit in their Jr. High/Middle school physical science prep. We did have a whole year on earth science, so I don't know if the typical British 14 year old could talk dew point charts with you, but I could have at that age. Some HS students do take the equivalent of A levels with their AP courses and tests. But I don't doubt our chemistry/biology/physics classes are at least on par with British standards, and most students take at least the first two if they are planning to go into higher education. But our undergraduate courses are four years and we don't do separate "college". In my school we had very vague requirements our first year, declared our "concentration" at the end of our second year, and started meeting with an advisor our third to write our senior thesis.
