I’m pretty sure Yeshiva Ketana of Passaic and possibly other large schools already do this, presumably for logistical reasons.
FTR, that would completely backfire, because if you’re really separating them because the older kids will do better, once the classes are separated, the older kids will continue to do better and the gap will only get even bigger. I happen to be a year-end baby and if I’d ended up in the weaker class or the class below I would’ve gone BALLISTIC long before now.
And Gladwell is a total hedgehog. He’s a fantastic journalist but I hate his books.