I agree with this WSJ op ed by two folks from Democracy Builders that charter schools, like all public schools, should have to backfill:

Many charter authorizers nationally allow charter schools to set limited "entry points"—typically kindergarten, fifth or sixth grade, and ninth grade. Lotteries, which by law are held once a year, pull students for these grades only. Because of space limitations, most applicants don't end up winning a seat.

For example, the New York City Charter School Center estimated that parents submitted 212,500 applications for 21,000 available spots last year. Even worse, since so few charter schools backfill, students who weren't in specified entry grades or who moved to New York midyear never even got a shot.

Charter schools that don't backfill are choking off an already limited supply of places despite the soaring demand. Between 2006 and 2014, according to a new analysis by our organization, Democracy Builders, New York City charter schools lost on average 6%-11% of students each year across grades, creating thousands of seats for new students. In 2014, for instance, at least 2,500 seats were freed up in third through eighth grade alone. Instead of filling these seats, most charter schools let them remain empty.

Why would charter schools not want to serve as many students as possible? Perverse incentives. Thanks to No Child Left Behind, there is a relentless focus on a school's percentages of proficient students, and schools that boast high pass rates make easy media exemplars. Without backfilling, a school can maintain the illusion of success; by maintaining or increasing the absolute number of proficient students while decreasing the number of total students, the percentage of proficient students—who have already had the benefit of charter schooling—is likely to increase.

High-need students who hope to enter a school in a non-entry grade are more likely to be transient, academically behind, homeless, new immigrants or English-language learners, so topping off school rosters with this demographic may hurt a school's "percent proficiency." Yet these are exactly the students who most need access to high-quality charter schools.