First, I am discussing low-SES schools in the US. It can be very different in other countries, especially Asian countries (first hand experience seeing this).
That said, in the US, many people (Asians included) can go to shitty schools and still succeed. Their success is despite the formal education they got at a low SES school, not because of it.
Generally when you see elite school success from students from low-SES areas, it is due to substantial intervention at home and/or extracurricular education/tutoring.
You ask why it is a handicap. If they could get quality education during school hours, their extracurricular hours could be used for deeper study or more interesting endeavors (e.g., research projects).
There are lots of Asians that grow up poor in the US and excel at far higher rates than poor African-Americans. There isn't really much evidence that SES has a large impact on things like SAT scores, I doubt it would affect being able to do well at Harvard (which I've heard is actually very easy once you get in).
I don't see what's wrong with suggesting genes are causing the gap. I'm not "dogwhistling" anything, this is the most powerfully supported scientific explanation. Science denialism seems to be extremely common on this forum. Genes definitely cause other gaps like sports gaps (or do you seriously believe that is because of racism in favor of blacks?) and are known to play a huge role in ability. Even if races have equal ability, do you seriously believe the extreme selection process for elite immigration won't affect outcomes?
That said, in the US, many people (Asians included) can go to shitty schools and still succeed. Their success is despite the formal education they got at a low SES school, not because of it.
Generally when you see elite school success from students from low-SES areas, it is due to substantial intervention at home and/or extracurricular education/tutoring.
You ask why it is a handicap. If they could get quality education during school hours, their extracurricular hours could be used for deeper study or more interesting endeavors (e.g., research projects).