A major difference is tiering in secondary education in Europe. There's not just a single "high school" degree, but there are vocational degrees (which you usually finish after 9th or 10th grade, after which you continue at vocational schools) and academic degrees (which you usually finish after 12th or 13th grade).
The academic degrees, since they only have to target a subset of students, can then be considerably more demanding than a more generally accessible high school degree (e.g., Calculus being a mandatory subject in some countries rather than the equivalent of an AP course) and you will cover a lot in school what in the US you'd do in the first and second year of college. For the same reason, Bachelor degrees in Europe are typically three year rather than four year affairs.
However, that is only part of the story, and a full undergraduate degree in the US (at a good university) will offer you a broader education than what you get out of most undergraduate degrees in Europe; if you want a broader education than what European universities offer, many European countries will point you to affordable continuing education. For example, if you want to learn another language in Germany, many Volkshochschulen offer courses for 2-3 Euro per hour of instruction (more if you need it to be credentialed).
Or, more simply put, while you can learn the same things in either the US or Europe, the educational content is packaged in somewhat different structures. And it isn't that one is better or worse than the other; the differences exist more as a result of historical accident or because of how education interacts with the rests of society's institutions and structures.