Disclaimer: I graduated from one of the top math high schools in Moscow in early 90s, I have MS in Math from Moscow State University, and a half finished PhD from the same place.
I think on average the difference between a high school student from Russia and US is negligible. Either country doesn’t really require much to finish school and many kids just do the tests without understanding what is going on. What separates Soviet Union (and now Russian) system is the practice of selecting kids with interest in mathematics / physics into special classes or schools for gifted kids. This government program feeds the mathematics and physics departments in the universities and allows kids who go through this program to get a very early start in mathematics. This is very similar to government sponsored sports programs with talented kids getting help with summer camps, or even all year round training schools.
I think on average the difference between a high school student from Russia and US is negligible. Either country doesn’t really require much to finish school and many kids just do the tests without understanding what is going on. What separates Soviet Union (and now Russian) system is the practice of selecting kids with interest in mathematics / physics into special classes or schools for gifted kids. This government program feeds the mathematics and physics departments in the universities and allows kids who go through this program to get a very early start in mathematics. This is very similar to government sponsored sports programs with talented kids getting help with summer camps, or even all year round training schools.