Mathematics in high school is linearly ordered, for example you need to take algebra before calculus, and calculus is (usually) the end of the road.
Things are very different in college. Sure, many subjects have prerequisites, but you cannot neatly order the subjects anymore. Things like abstract algebra and differential geometry are pretty independent. You can take both at the same time, or you can take one much later than the other (or not at all). So in fact, there are highest levels of math is many directions, there is not just one level of math that is the highest. For example, set theory and forcing might be one "highest level class", but "algebraic geometry" might be another.
Also note that after a while, you are expected to self-study subjects, they don't get taught anymore.