The difference between American and Soviet schools back in the day was that Soviet curriculum was designed for ~80% to pass, while American was designed for ~99% to pass. (Nowadays, Russia has lowered the bar significantly, so the difference is no longer as huge.)

An average student who finished 5th grade in an average school in USSR could start 6th grade in the U.S. and coast along on what they'd learned until the first year of high school.