They are not. I am not from Slovakia, but Albania (a neighbor country) and I can say that the average public high-school in the US is not any better than most

They are not. I am not from Slovakia, but Albania (a neighbor country) and I can say that the average public high-school in the US is not any better than most high-schools in south-eastern Europe. But, graduating from an US high school gives you much higher chances to get accepted to a US college, and get a scholarship (if you have top grades). Talking this from experience: I came in the US for my senior year of high school as an exchange student (lived with an american family), and then got a full academic scholarship from an american university. I was taking mostly AP classes, but still found my self running circles around most students in sciences (math, physics, chemistry) etc, and I found generally much easier experience than my classes back in Albania (even with the language handicap I had at the time). But still, that year in the US was very valuable in learning language, culture, making life-long friends, and getting me into college with a full scholarship.