As rancorous charter school space debates continue to dominate the headlines, another lower-profile round of discussion about who attends the schools is just beginning.

Charter leaders strategizing about how to work with Mayor Bill de Blasio’s administration and avoid paying rent say that they believe committing to particular enrollment policies could be one way to assuage de Blasio’s and Chancellor Carmen Fariña’s concerns about charter schools “doing their part.”

One main issue is backfill, or what happens to space vacated by students who leave charter schools. Some schools, seeking to fulfill a larger mission and bolster their finances, fill those spots by calling students off of their waiting lists. Other schools focus on teaching the students who remain, avoiding a potential drop in test scores and the social and academic disruption of adding new students.

The debate over which policy is best has long divided the charter sector, as critics have charged that schools that do not backfill are not serving their share of high-needs students.

Now, the issue is growing in prominence as school leaders try to anticipate how the mayor will deal with charter schools in the years ahead, and especially how the city might charge charter schools rent to operate in public space. Meanwhile, financial pressures on schools already paying rent have made backfilling a necessity for a growing number of schools.

De Blasio’s meeting with a coalition of 34 charter school leaders on Monday — city officials’ second sit-down with them in two weeks — didn’t get into detailed policy discussions, though attendees said the conversation touched on how to use enrollment policy to promote equity. But the charter leaders are now planning regular meetings with city officials and say they expect the backfill issue to resurface.

“I think facilities is probably the bigger one,” Future Is Now founder and coalition member Steve Barr said of the issues on the table. “But they might say, these are some things that we need assurances on.”

Some charter leaders outside the coalition agreed. “It’s definitely more of a public policy issue than it has been in the past,” said Steve Evangelista, co-founder of Harlem Link Charter School.

One of de Blasio’s few points of leverage over charter schools

Charter schools, which are publicly financed but privately managed, sit outside the mayor’s authority. De Blasio does have the power to set the conditions by which charter schools can operate in public space, though. Governor Andrew Cuomo said as much last week when he asked, “The question becomes, what should the criteria for co-location be?”

The city has laid out some criteria for next year’s co-locations, but they were logistical considerations like the ages of the students who would be sharing a building and the proposed school’s size. The mayor has yet to say how he will make decisions about charging charter schools rent, a key campaign promise.

Requiring certain enrollment practices like backfill could make sense, given Fariña’s criticism of charter schools that she says don’t serve similar populations as district schools. Last week, she criticized Success Academy CEO Eva Moskowitz for implying that there are certain high-needs students “she cannot help, necessarily, because she doesn’t have the resources for them.”

One charter leader described the potential trade-off this way: The city provides space rent-free if the schools commit to more inclusive enrollment tactics. Then the choice becomes the operator’s: do we want to go along, or stick to our model and pay a penalty for it?

For Stacey Gauthier, principal at Renaissance Charter School, the decision to backfill in every grade wasn’t really a decision at all. Like other charter schools, her school has waiting list of students, which many schools cite as evidence of their success and demand.

“It was just natural, just organic,” Gauthier said. “You have a space, there are 2,500 kids on a wait list—why would you not fill the space? It never crossed my mind—’Wow, don’t backfill because you might have to work harder to make that kid a Renaissance kid.'”

“My charter colleagues should really look closely if their enrollment practices don’t look equitable,” she said.

Backfill comes at a cost to schools

Others, including some coalition members, argue that the choice is more complicated. In 2012, the New York City Charter School Center put that ideological divide mildly. “NYC charter school leaders have mixed opinions about backfill enrollment,” its State of the Sector report said.

Backfilling seats that open up can pose steep challenges for schools. Students who enter the school midyear or at one of a school’s higher grade levels can have trouble adjusting to a new school and be academically behind. Midyear entries especially are more likely to have unstable home lives, leading to them leaving the school—meaning that one “backfilled” seat might actually be filled by two or three students over the course of a year.

Research has shown that students who leave charter schools tend to be lower-performing academically, so not replacing them can boost scores overall — a move that benefits charter schools that are eager to prove their value.

“On one hand, why should they?” said Gary Miron, a professor of education at Western Michigan University. “It’s a real disruption for the classroom teacher. Traditional public schools are plagued with this problem, especially in high-poverty areas where there is lots of student attrition.”

An understanding of those challenges and a desire to maintain a particular school’s culture has led some charter school networks to reject backfilling, especially in the higher grades. Success Academies only backfill through the third grade, and students in all subsequent grades up to high school must have started by that grade.

Success Academy has said that their longer school days and years help students jump so far ahead academically that placing older students in without that background would be unfair to them. Last year, 58 percent of its students were deemed proficient in reading and 80 percent in math on state exams.

“We want all children to feel and be successful. We wouldn’t want the newer children to be at a disadvantage,” a Success spokeswoman said last year.

Actually getting seats filled can also be no easy feat.

Charter schools face the question of whether to hold back incoming students who are behind academically or insert them into the grade they were expecting to attend, knowing that families might not accept a spot for their child if it means he or she is held back a grade.

And since state law requires charter schools to admit students by lottery, a school must start at the top of its waiting list and contact families until one accepts to fill a seat that becomes vacant. That process can take time if families have gotten comfortable at the school their child attends, making the choice to fill seats a costly one logistically. In contrast, the city assigns students to district schools, which do not have to use their own resources to fill seats.

It also has ideological and material benefits

Despite the challenges, charter schools have good reasons to fill seats that become vacant.

District schools take students after their entry grades and many accept students mid-year, and not doing so raises questions about whether charter schools are doing enough to educate a fair share of high-needs students. In addition, it complicates performance comparisons between charter and district schools — which the charter sector cites as a justification for growing – if the schools follow different enrollment rules.

Two weeks ago, the chair of Harlem Link Charter School’s board of trustees asked a question of the room during a board meeting: Who thinks it’s in the best interest of the community to continue the school’s backfilling policy?

It was a loaded question. Evangelista, the school’s co-founder, has spoken openly about his school’s policy of accepting new students at every grade and its connection to the school’s relatively lower test scores. Last year, 17 percent of its students cleared the state’s proficiency bar in reading, below the city average, and 29 percent did in math, which is at the city average.

“What is the community? Is it just the school community, when it’s very clear that bringing kids in that don’t know our school and its culture is very clearly detrimental?” Evangelista asked.

He has answered that question with a no, for now, and Harlem Link is choosing to define “community” as the broader neighborhood and school system.

There are financial as well as ideological considerations. A district school that loses a student after Oct. 31 keeps its funding for that student, but a charter school loses funding for a student as soon as the student leaves. Allowing more seats to remain unfilled each year exacts a steeper toll on school budgets.

For many charter schools, especially those in private space that have to pay rent, budgets are so tight that operating at anything but their highest capacity makes the school unsustainable. With prospective charter operators not counting on generous offers of public space from the de Blasio administration, backfilling may become more common.

Judi Kende, who works with prospective charter school operators through the Low Income Investment Fund, an organization that finances charter school facilities, has seen that trend firsthand. When LIIF is assessing a charter school’s financial viability, backfill policy gets special attention, Kende said.

“People are doing more backfilling. They kind of have to if they have real estate space,” Kende said.

For now, a mix of policies

Charter schools must spell out their enrollment policies when they ask for permission to operate. But authorizers have been loath to require charters to adopt one backfill policy or another, seeing it as one way in which the schools exercise the autonomy that defines them as a charter school, and so schools frequently include vague language in their charters.

“In terms of just replacing students, we leave that up to schools,” said Susan Miller Barker, the head of SUNY’s Charter School Institute. (The institute does plan to require more explicit descriptions of enrollment policy from new applicants, it announced in January.)

That leaves the city’s charter schools with a patchwork of policies, and some have changed over time. Democracy Prep and Explore Schools both accept students in all grades, for example, as do Beginning with Children II and Brooklyn Excelsior Charter School, while Harlem Village Academy doesn’t backfill upper grades.

In a Nov. 2009 report in Education Sector, Achievement First CFO Max Polaner said “the dream was to bring in kindergartners only,” given the difficulties of backfilling. The network’s application for the K-8 Achievement First Central Brooklyn, set to open next fall, notes that it will backfill up to the eighth grade.

For now, the coalition of charter schools sitting down with city officials is thinking about a number of policy issues to tackle down the line, with facilities and enrollment at the top of the list, Gauthier said.

“They’ve said, ‘We want this to be two-way, we know these are items that need to be addressed. We hope you can be our thought partners in this,’” she said.

De Blasio could draw inspiration from Massachusetts, which required all charter schools to backfill in some grades beginning in 2010, or from Denver and New Orleans, which use universal enrollment systems to ensure that charter and district schools follow the same policies. But De Blasio has so far not prioritized enrollment policy when discussing charter equity issues, and Evangelista cautioned that there are many possible policy priorities for City Hall.

“The policy debate right now is a sprawling octopus, and his office has the potential to drive attention to one or two things,” Evangelista said. “It could be testing, English Language learners, special ed, recruitment—these are all hot buttons.”

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This story has been updated to clarify the difference between district and charter school funding over time.