Employees at the International High School of New Orleans voted 26-18 in favor of unionizing Friday, becoming the third New Orleans charter school to do so in the past few years.

Teachers and paraprofessionals opted to join up with the United Teachers of New Orleans to negotiate a collective bargaining agreement. One vote was challenged, but it won’t be counted because it wouldn’t affect the outcome.

“It’s a great day for teachers in the city of New Orleans,” UTNO President Larry Carter said.

The vote followed tense debate among local educators about UTNO’s recent push to unionize some of the city’s charter schools, nearly all of which operate as independent nonprofits without collective bargaining contracts.

Union supporters wore small buttons, while opponents wore T-shirts that read: “If you CAN READ, thank a teacher. If you cannot, thank a teacher’s union.”

Only one New Orleans school has successfully negotiated a collective bargaining agreement for teachers since Hurricane Katrina, which cleared the way for a state takeover of most public schools in the city and the end of the citywide agreement that UTNO had with the Orleans Parish School Board.

Benjamin Franklin High School voluntarily recognized the union and concluded contract talks last spring.

Last year, Morris Jeff Community School’s board voluntarily recognized a union as well. A school administrator said Friday that negotiations over a contract are ongoing.

The union drive at International High School was announced just days after teachers at Lusher Charter School began a similar effort. Teachers at Lusher voted against unionizing last week, though a smaller group of paraprofessionals voted the other way in a separate ballot.

Teachers at all four schools have had support from UTNO, which is affiliated with the American Federation of Teachers. The national union has led a recent organizing push in the New Orleans area, also investing heavily in 2014 Jefferson Parish School Board races.

Some school administrators have been determined to resist the union effort. A day after the Lusher vote, International High School’s principal and board chairman sent an email to staff members saying they would have “an opportunity to let UTNO know it has no place at IHSNO either.”

The union had asked International High’s nonprofit board to voluntarily recognize UTNO as the bargaining agent for its teachers. But the board never formally responded, leading the union to request a vote supervised by the National Labor Relations Board.

Whether that board has jurisdiction over the city’s charter schools has been up for debate. At a hearing two weeks ago, the school’s attorney, Brooke Duncan III, argued the charter is a public school created by state law and therefore not subject to the federal agency’s authority.

UTNO lawyer Louis Robein argued the charter is a private nonprofit that operates under an agreement with the state, meaning it is a private employer and subject to the board. A regional NLRB director ruled in favor of the union, leading to Friday’s election.

Teachers cast their votes during a three-hour window that ended at 11:30 a.m., then gathered in the school’s community room for vote tallying. Union representatives, International High board members, Duncan and others joined the labor board agent in tallying the votes.

UTNO now will try to negotiate a contract with the school’s board on behalf of International High employees. The contract will be unique to the school.

The school could continue to challenge the labor board’s jurisdiction, as Lusher’s administration did in a brief filed Wednesday.