Teachers at the Pennsylvania Virtual Charter School in King of Prussia have unionized.

After a long wait that included a controversial decision by the National Labor Relations Board, ballots showed that teachers at the school voted, 57-15, in favor of being represented by the PA Virtual Education Association, an affiliate of the Pennsylvania State Education Association.

Teachers voted in 2015, but the ballots were immediately impounded when the school challenged the NLRB's jurisdiction.

Late last month, the NLRB voted, 3-1, to uphold its jurisdiction. The ballots were opened and counted Wednesday.

The NLRB said it had oversight of the election because charter-school teachers are employed by private corporations, not public school districts. The board said that while charter schools are paid for with tax dollars, they operate like government contractors.

The board made a similar ruling at the time in a case involving a charter school in Brooklyn, N.Y.

Pennsylvania Virtual enrolls 2,320 K-12 students across the state who receive online instruction in their homes. It is the third cyber charter in Pennsylvania where teachers have unionized.

On Thursday, the school issued a statement noting that the NLRB had ruled it was a private corporation. "We respectfully disagree with this decision," the school said.

Officials said the cyber charter had been authorized by the Pennsylvania Department of Education, enrolled students from across the state, and was held to the same standards as public schools.

In addition, the school said its board members and administrators were considered public employees under state law.

"We respect the right for our staff, who are public employees, to organize under a collective bargaining agreement," the statement said. "However, concerns remain regarding our public school being categorized as a private employer."

Kevin M. McKenna, an attorney who represents Pennsylvania Virtual, said its board would review the NLRB ruling this month.

Pennsylvania Virtual was not the first charter in the state to have an election overseen by the NLRB.

In 2013, the now-defunct New Media Technology Charter School in Northwest Philadelphia and an affiliate of AFT Pennsylvania held a union election under the auspices of the NLRB rather than the state labor board.

New Media sought the NLRB's involvement as a result of a 2012 board decision that a charter school in Chicago was not a public school because it was founded by a nonprofit, was overseen by its own board, and was not part of state or local government.

martha.woodall@phillynews.com 215-854-2789 @marwooda