Can't we agree that both extremes are wrong? While I agree that it is wrong to assume there is no such thing as innate human ability, and it is wrong to assume everyone can achieve equally, you seem to be arguing the opposite; that there is nothing that can be done to improve achievement for those who are struggling.
This simply isn't true. There are things that can be done to improve the outcome for students, and we should continue to work to try to improve the success of all students. This doesn't mean that you expect everyone to achieve equally, just that you can help people achieve more than they would have without the help.
I also find this argument a bit paradoxical; if you truly believe that innate ability is the only determining factor for how well students do, then why do you worry about handicapping those who are capable? It shouldn't matter if we force them into classes they are too advanced for, since how we educate them doesn't matter and only natural talent matters.
It seems that you believe schooling does affect achievement, since you want to make sure we aren't holding back the high achievers, yet you are saying at the same time we shouldn't worry about how we educate the low achievers because they are stuck where they are no matter what. You can't argue that it matters for high achievers but not for low achievers, that doesn't make any sense.