I went through eastern block's educational system (elementary and high school), the revolution caught me as I started university, I do have personal experience.

Three factors were at play: 1)Not much dumbing down, if you don't get it, you will barely scrape through. The system was not primed for holding back everybody down because of a dumb minority.

2) A dedicated effort to identify the smart kids, direct them to focused high quality schools, encouragement to get involved in serious competitions (mathematical olympiads, but also correspondence seminars) and math clubs.

3) Not so much distracting options to do other interesting things (very unlike the west), so much easier to actually attract and keep the smart kids interested.

The education system is in much worse state now - both 1) and 3) are gone, and 2) is severely affected, mostly by 3).