Mike Petrilli makes a strong argument for why charter schools should NOT be required to backfill. I'm not persuaded – I think we gotta play by the same rules as regular public schools in this area, even if it's disruptive to a school's culture – but I respect his arguments:

In today's Wall Street Journal, Princess Lyles and Dan Clark, the executive director and lead organizer of the school-choice group Democracy Builders, argue that states and/or authorizers should require charter schools to "back-fill" their "empty seats" when they lose students to attrition. This is a terrible idea.

Their argument in favor of requiring charters to backfill is twofold. First, they say it's unfair to compare schools that backfill to those that don't, because those that don't (like Eva Moskowitz's Success Academies*) almost certainly end up with a more motivated, higher-performing population over time as weaker, less engaged students depart for less challenging environments. It's especially unfair, they say, if the comparisons are made on proficiency rates—the percentage of students passing state tests—instead of individual student growth. (I agree that such comparisons are unfair. More on that below.)

Second, they argue that, by not backfilling seats, schools like Success Academy are limiting opportunity. As a result of this policy, parents only have a shot at getting their kids into schools at designated entry points (like kindergarten or sixth grade). If families lose the charter school lottery for those specific grades, they are out of luck forever.

It's a reasonable point, and I respect schools like Democracy Prep that welcome in students at any grade when space opens up. But whether to do this should remain the prerogative of the school, not the state or its regulators.

Why? Because there are strong instructional arguments for not backfilling. Great schools spend a lot of time building strong cultures—the almost-invisible expectations, norms, and habits that come to permeate the environment, such as the notion that it's cool to be smart and it's not OK to disrupt learning. Culture-building is a whole lot harder to do if a school is inducting a new group of students every year in every grade. Furthermore, schools that help their charges make rapid gains in their early years will be forced to spend a lot of time remediating new students who enter midstream. That's why so many solid charters and networks that launch as middle or high schools eventually reach down to start serving students at age four, five, or six. It's hard to remediate a kid who has already gone through half a dozen years of learning nothing in a dire school.

This isn't just a technical challenge; there's a moral question too. Backfilling is surely good for the student who gets to claim an empty seat. But what if it's bad for their new peers? What if the disruption to the many outweighs the benefits to the few?

By the way, KIPP in NY backfills every grade through 10^th grade.