A LEARN charter school (right) rents space across the street from the now vacant Calhoun North school (left). Chicago Public Schools paid $67,151 in utilities for Calhoun North from Sept. 2013 to July 2014, according to data obtained through a Freedom of Information Act Request. At the same time, CPS pays LEARN $750 per student to offset rent and other facility costs. (WBEZ/Becky Vevea)

There are 40 school buildings still sitting vacant across Chicago since the mass closings of 2013. Just two have been sold and the rest cost Chicagoans $2 million annually to maintain.

These schools are slow to sell for a number of reasons. Many aren’t in thriving neighborhoods. The buildings are old. There aren’t a lot of obvious alternate uses.

But one big reason the empty schools continue to collect dust and fall into disrepair is this: CPS chief Barbara Byrd-Bennett, who is currently on leave, made a promise that eliminated a whole group of potential buyers.

Map: How close are charter schools to vacant CPS buildings?

“We currently cannot sell any of the properties to a charter school,” said Mike Nardini, the district’s real estate agent. “Does it limit our buyers? Only to the extent that it can’t be a charter any more than it could be a nightclub.”

The promise made sense at the time considering one of the main arguments for shutting down 50 schools was to downsize the district. CPS officials argued the school system was operating inefficiently with too many schools and not enough students enrolled.

But the Chicago Board of Education continues to authorize new charter schools. In the past, charters often moved into closed school buildings, but that upset many community people, who saw the publicly financed, privately operated charters as replacing traditional neighborhood schools.

CPS spokesman Bill McCaffrey said Wednesday the Board could be convinced to change its mind.

“If a community were to determine that they do want a charter school in that closed site, then that is something that we would consider,” he said.

McCaffrey was very careful to say officials would break the promise only if the community supports it, not because it might save money.

“Our first consideration isn’t the financial implication,” he added.

But saving money is the biggest problem CPS has right now, and the ‘no-charter’ promise complicates things. Charter schools that are in private buildings currently get $750 per student from CPS to offset rent and other maintenance costs. This is commonly known as a “facilities reimbursement.”  And while these real estate deals can be complicated, the bottom line is that Chicago taxpayers end up paying extra to charter schools who are forced to rent on the private market.  And those same taxpayers also are paying to maintain buildings the city already owns, but isn’t using.

“These are assets that we have in our city that are paid for typically and what we don’t need are more vacant buildings,” said Andrew Broy, executive director of the Illinois Network of Charter Schools.

In many cases, the charters and the vacant buildings are just blocks away from one another. In Garfield Park, a LEARN charter school rents space across the street from the now vacant Calhoun North school. In Woodlawn, a University of Chicago Charter School is planning to build a brand new school on a plot of land right next to a CPS-owned building where it currently operates.

It all speaks to a very basic and fundamental question that no one—CPS, the mayor, city aldermen—has grappled with: Exactly how many public schools does Chicago need? And where should they be?

When asked after Wednesday’s City Council meeting, Chicago Mayor Rahm Emanuel said that’s not his job.

“That’s something CPS will do based on the student population, patterns of growth,” Emanuel said. “That’s a fair question, but not the only question. Are the schools that are open achieving educational excellence?”

CPS is holding public hearings Thursday night on new requests by charter schools to move to different locations. Most have plans to move into private buildings, but at least one, The Chicago Tribune reports, wants to move into the closed Peabody Elementary school on the West Side. Peabody was sold last fall.

Becky Vevea is an education reporter for WBEZ. You can follow her @WBEZeducation.