You provided anecdotes that some people do not perform as well as they should on tests. I don’t doubt that.
However, my question still remains: How should colleges compare students from a high performing school to a low performing one? Certainly some high performers can come out of low performing schools, but statistically they are much more likely to come out of the high performing school. Why should the students at the high performing school be penalized for this given the more rigorous competition they face?
Now that my D is a junior, I am starting to look at Naviance scattergrams. What I notice is that highly selective colleges seem to have fairly hard cutoffs for grades but fairly soft cutoffs for SAT scores. For example, almost nobody has been admitted to Harvard from our school with a GPA lower than 4.5, but there seems to be no difference between a 2200 or a 2400 in terms of admission or rejection. This is a limited view, but it suggests that selective college use SAT scores to back up the grades.
I would also note that when it comes to standardized testing, US students have it fairly easy. Students can take the SAT or ACT (or both) multiple times, on dates they prefer, anytime from freshman year onwards. This is very different from what most of the rest of the world does, which is that a single test determines your future.