My dad was similarly hyper-tracked: he entered grade school at 7 and graduated at 16 (he grew up in the Philippines, but as part of the expat Filipino-Chinese community, and went to Chinese schools). When my dad tried similarly hyper-tracking me academically (suburban Boston), I got beat up, physically, by my peers. Having been traumatized into becoming a terrible student, I later found out I did much better economically than many of my more academically-diligent college classmates that ended up getting physics Ph.Ds.
The U.S. is fundamentally an exchange economy: you succeed based on how well you can make other people happy. That privileges EQ and social intelligence over IQ and academic intelligence. The USSR was a command economy, and China was an export economy until very recently. Both of those privilege having specific technical skills in areas that are particularly useful to the state, many of which have mathematics and science as a precursor. It wouldn't surprise me if China starts devaluing academic achievement as it transitions from an export economy to an exchange economy. We're already starting to see it with the fuerdai, the children of the new Chinese rich.
This is a problem. Regardless of whether hyper-tracking is beneficial, no kid should have to suffer from this kind of bullying. I don't know why bullying in the U.S. is (or seems to be) so much worse than other countries.
There are many reasons. Parents and students wield too much power in the American school system, and the idea of tying a school's funding to local property taxes makes this worse.
In addition, America lacks the cultural norms of respect and deference for education and educators that are present in more traditional societies like India or China.
Asians always address their parents with words meaning "mother" or "father". Calling them "you" is extremely rude.
We can go on and on how vastly different the norms on respecting parents (and teachers).
There were programs after hours for gifted students but for the first eight years normal hours you were locked in with the other animals there wasn't anything special and it was living hell. Don't think I call my young peers animals for nothing, I do not only carry the emotional scars but also the physical: more than thirty years later it's visible where one of them has bitten my shoulder at the age of 13. I ran away from the school very early once and my father practically dragged me back and I was contemplating suicide when I was 9.
Doing a lot of those after hour programs and an ungodly amount of luck and goodwill made me pass the admittance exam of the best special maths high school class in Hungary. You know those question where they ask what you would do differently in your life if you could do it again? Well, I would tell 13.5 yrs old me to do the goddamned homework for the preparation course! One of the homework questions was the entrance exam and I mostly fubbed it and as I learned much later one of the two teachers didn't want to admit me while the other saw something... It makes me shudder to think of my life if I hadn't gotten in that class...
I went to a top university in the US and there were definitely kids who arrived being much more advanced in certain areas such as mathematics than the rest of us. Maybe it is not as regimented in the US but there are definitely highly talented kids who take much more advanced STEM classes than the average student.