Four charter schools are opening in Boston next fall and they need classroom space. The Boston Public Schools are downsizing and will have 10 empty buildings at the end of this school year. It seems like a simple case of matching supply and demand, where both sides could benefit from striking a deal. But nothing is that simple in Boston.

Boston Mayor Thomas Menino became a convert to charters during last year’s mayoral race and again when the city was trying to land Race to the Top federal education funds. But it’s clear he’s not sold completely and feels charters and city schools are not competing on an even playing field.

Dot Joyce, the mayor’s spokeswoman, says Menino is not opposed to leasing empty city schools to charters, but he wants a commitment from the charters that they will diversify their classes to include more English language learners and children with special needs.

“If you’re going to use a former public school building, we have to have an agreement that they teach every type of student we have,” Joyce says. “It’s a big conversation. There’s a lot to it. It’s not just about buildings.”

The conversation is starting to take place. Marc Kenen, executive director of the Massachusetts Charter Public School Association, told his members in an email last week that he had just met with Boston Public Schools Deputy Superintendent Michael Goar, who for the first time shared with him a map showing the city’s empty or soon-to-be-empty school buildings.

Kenen said there is no guarantee any of the empty schools will be leased to charters because the city may need some of the excess space for its programs. But Kenen said he was encouraged that Goar was finally discussing the process of leasing schools. He also urged his members to keep quiet about the discussions for fear of alienating parents and other groups already upset that neighborhood schools are being closed.

“I ask you to PLEASE keep this confidential!” Kenen says in the email. “This process is very encouraging, but sensitive and tenuous, so while the existence of vacant school buildings is certainly not confidential information, a public disclosure that the district is discussing this with us could jeopardize the entire process.”

School officials say the leasing of empty school buildings is part of a much larger conversation they are having with charter schools. The officials say they are crafting a district-charter compact covering collaboration on a variety of fronts, including a common student registration and assignment process, joint operation of facilities, and more cost-efficient transportation. (The Boston Public Schools pay the cost of transporting students to charter schools no matter how far apart the school and the student are.)

Several charter school officials, who asked to remain anonymous, were not optimistic about coming to a lease agreement with the city in time for this fall. These officials said they will probably have to use less attractive warehouse space, retrofit it for classrooms, and make do. It will cost them more money and it will mean lease payments that could have gone to the Boston Public Schools will instead go to private interests.

At a City Council hearing in March on the school closings, Goar acknowledged leases can sometimes be done more quickly. He said the school system two years ago took only three months to lease the empty Hamilton school in Brighton to the private Bais Yaakov Jewish Day School. The Jewish school pays the city annual rent of $62,150 and covers utility and custodial costs.

“Hamilton created a unique opportunity,” Goar said in explaining the quick turnaround. “It was a religious school that typically doesn’t compete with BPS.”

City Councilor John Connolly seized on Goar’s remark and said the reason the city is dragging its feet in leasing empty space to charters is because it considers them competitors. Goar acknowledged city schools are facing competition, but he said the school system is carefully planning what to do with its empty space and also wants to negotiate a better working relationship with charters.

“It seems like a smokescreen to me,” Connolly says. “BPS is trying to run out the clock and they have no intention to lease to charters. The sad part is we hear so much about BPS financial difficulties, but they don’t want to take advantage of the revenue stream they would get from leasing the space.”