The reality is that most studies show that charter schools shed their low-achievers, which go straight into the public school system.  Though there is attrition in the public schools as well, their low-achievers do NOT go to charters but into other public schools.  The number of students who test into schools like Hunter HS is vanishingly small.
 

I think when you “backfill” as district schools and some charter schools do, you also have to spend more time getting new kids caught up on your routines and procedures – from classroom behavior that promotes learning to things like notetaking strategies, you spend more time on these in later years than you would if you only had kids who had started in your program. In my experience, this is more time-consuming than you’d think, and it’s time you’re not spending per se on helping all kids grow. (Of course all those things need reinforcement for everyone, but still…).

What amazes me is how any school can afford NOT to backfill.

That said, I have a major beef with his description of the way the backfill argument is used by so-called ‘anti-reformers,’ or even rank amateurs like me who simply don’t buy into the charter agenda lock, stock, and barrel. Mentioning the failure to backfill is an absolutely valid and appropriate defense to charter braggadocio about how they educate the exact same kids, and it’s an especially salient counterpoint to charter complaints about space and equity–why in the world should any network or operator get more schools when as a matter of policy they don’t fill every seat in the schools they already do have? 

It would be difficult to force charters to educate ELLs, free-lunch-eligible, and special ed students in the same proportions as their home districts. It would be incredibly easy to prohibit them from not backfilling, and I hope this is something that’s looked at by the legislature.