A Chicago charter school that has received more than $23 million in public money since opening in 2004 is arguing that it is a private institution, a move teachers say is designed to block them from forming a union.

In papers filed with the National Labor Relations Board, attorneys for the Chicago Math and Science Academy on the city's North Side say the school should be exempt from an Illinois law that grants employees of all public schools the right to form unions for contract negotiations.

The school of about 600 students is appealing an unfavorable decision by a regional director of the national labor board. Academy officials say charter schools don't have the governmental ties that characterize public schools, such as government-appointed leadership or controls over wages, hours and working conditions. In other words, they say, the same freedoms over personnel and policy that many credit to charter schools' success are also indicative of their independence.

It's a provocative argument, particularly in Chicago, where organized labor is woven into the city's social and political fabric. Education experts say this dispute between the academy and its teachers — believed to be the first to test a charter school's independence at the national level — underscores a rising tension between charters and traditional public schools coexisting under Chicago Public Schools, the nation's third-largest school district.

"They have been guarded and protective, and the prospect of a labor contract is a risk to those basic freedoms," said Timothy Knowles, director of the Urban Education Institute at the University of Chicago. "That's why charters have been reluctant to go there and have fought efforts to organize."

In the last decade, charter schools, which have relative autonomy over decision-making but rely heavily on public funding, have become a force in Chicago's crowded public school system. Proponents say charters' greater flexibility over policy, resources, the hiring and firing of teachers, and the length of the school day and year has made them education innovators that have improved learning and college preparedness.

Part of their success, they say, comes from keeping organized labor at bay, although that is changing. Eight of the city's 38 charter schools have organized teachers who've negotiated and signed contracts, officials say, and four others are pushing to do the same. None of those battles, however, has escalated like the one at the academy.

"It's pretty clear (the school) is trying to do an end run around the Illinois Legislature," said Chicago Teachers Union President Karen Lewis. "And I do think having to go to the federal level has chilling effects because it stalls the negotiations for conflict resolutions. … If I was working in this place, I would be a little nervous about exercising any rights or any kind of voice."

The school's teachers say the academy wants it both ways: public when it's accepting millions each year from Illinois taxpayers, but private when it wants to assert power over employees.

"They're trying to stall, they're trying to break us," said Kate Ostler, 25, a third-year teacher at the school. "It seems like a game to them, when all we're trying to do is empower ourselves and to create a better learning environment for the kids."

The conflict has sparked tensions at the school, teachers said. School officials often use what teachers described as anti-union rhetoric in meetings with teachers (the school says it is not anti-union), and the school last year fired a popular and well-respected teacher who was part of the union effort, teachers said.

"All it's done is fueled our fire," Ostler said. "This is more than just a club full of teachers together. It's about our careers and about doing what's right."

The academy's principal, Ali Yilmaz, declined to talk about the union issue and referred questions to the school's attorney, James Powers. Powers downplayed any animosity between the two sides but acknowledged the importance of the labor board's upcoming ruling.

"They could have denied it, and it would have been the end of the story," Powers said. "Not only did they not do that, they also invited briefs for any and all interested parties around the country. That suggests that they think it's national in scope too."

The academy, 1709 W. Lunt Ave., in the Rogers Park neighborhood, was created six years ago by a group of educators and business leaders as an independent charter school with a focused curriculum.

More than 80 percent of the school's annual $5 million operating budget comes from CPS, records show. The rest is made up of state and federal grant money and private fundraising. Powers argues that public funding alone is not the mark of a public institution. The more important markers, he said, are that charter schools are established by private citizens and that government has little, if any, sway over its operation.

If it were solely about using taxpayer money, Powers said, "you could make the case that any governmental contractor who gets a pretty big chunk of their business from the government is a governmental entity."

However, the Illinois Charter School Law, crafted in 1996, clearly defines charters as public and open to any students who live within its geographic school boundaries. The schools are required to abide by all state public school laws and codes.

"Charter schools are public schools," Knowles said. "They are operated in most cases by private nonprofits, which is where the gray area lies. But if you back up a little bit, they're fundamentally public. They're public institutions serving public kids. They are not tuition-based. They are underwritten by taxpayers. They are responsible for meeting public school law and adhering to the regulatory environment."

Powers maintains that the school is not anti-union but is seeking clarification from the national labor board about which laws — state or federal — apply to its employees. Both sets of laws allow for the formation of unions, but federal law carries certain restrictions during negotiations.

Union advocates said that if the school can't figure out a way to block the union's creation outright, it's trying to delay the process to avoid negotiating a contract.

"They've chosen to spend tens of thousands of dollars on legal fees and deny the teachers their rights for months on end," said Sam Lieberman, an attorney for the American Federation of Teachers. "Judged by its actions, (the school) is clearly anti-union."