Eighty teachers and guidance counselors at the Advanced Math and Science Academy Charter School in Marlboro recently made history when they joined a union — but not the one you might think.

They did not join the Massachusetts Teachers Association, which represents 110,000 public school educators across the commonwealth. Nor did they join the Massachusetts Federation of Teachers, which has 25,000 members.

No, they joined Teamsters Local 170 in Worcester, a 4,000-member union of mostly truck and bus drivers and warehouse workers.

The AMSA vote was certified July 16 by the state Department of Labor Relations.

In so doing, the AMSA teachers became the first charter school teachers in the country to join the Teamsters. And they were only the third charter school in Massachusetts whose educators have voted to join a union.

In a press release from the union, two teachers explained why they chose to join the Teamsters. Like most charter schools, AMSA hires teachers to one-year contracts and does not offer seniority. The school did not, until now, have to wrestle with the union to fire employees.

"A lot of key people, award-winning teachers, were let go. That decision cannot be made in five minutes in a back room so someone else can get the job. We need a process so that everyone can feel more protected," said Lino Alvarez, a computer science and Web design teacher at AMSA.

"We looked at other unions, but decided the Teamsters was the best union for us."

Jessica Bowen, who has worked at the school for six years, said in the press release: "I see how vulnerable workers can be without unions. I'm a history teacher, so I studied the labor movement, but in an academic sense. To see how it works firsthand is eye-opening. The Teamsters were so positive and every time we asked questions, we got answers. It felt right, like, we now have the support we needed."

Pauline Ryan, vice chair of the school's board of trustees, said that all employees at the school have been one-year, at-will contracts since the school opened. She said that the board has agreed to examine the issue for the upcoming school year.

Teachers have always had a voice at the school, she said, through an elected teacher representative on the school's board of directors; through the chain of command leading to the executive director and the board; or, teachers could address the board directly in public session.

Teamsters organizer Paul Stuart said the teachers were fed up with the way the school administration has treated them and with the constant turnover, and the teachers called the Teamsters and asked them to help set up a union vote.

"They felt they were not being treated fairly, and so we got involved and started organizing there," he said.

He said the school administration fought "tooth and nail" against the teachers' union vote.

Ms. Ryan, of the school's board, said the school administration did not know about the union vote until it happened. The school administration did not favor a union, she said.

"The school administration did not believe that a union would be in the best interest of the school in the long term," she said. The board is preparing to negotiate with the union, she said.

"I think the union will be a good way for the teachers to come together and speak with one voice. We think that these negotiations will help us understand what the teachers' concerns are," she said.

If you think that it is strange that charter school teachers chose to join a union dominated by truck drivers, think again. Unions have become more diversified, partly out of necessity, as membership has shrunk since its peak 30 years ago.

Just to name a few examples: The Service Employees International Union long ago moved beyond food service and hotel workers and now represent many other other types of employees. Adjunct professors at UMass Lowell are represented by the United Auto Workers union, as are some research assistants at New York University.

"A lot of unions now consider themselves 'general workers' unions," said Clark University professor of labor relations Gary Chaison. "You won't see the union representing Major League Baseball players start to represent teachers."

But the Teamsters, auto workers union, steelworkers union and others like the Service Employees International Union once concentrated on one particular type of worker, but are increasingly willing to take on new types of members.

"It does make for some strange pairings," he said, citing one example of a communication workers' union that once focused exclusively on telephone operators and now counts social workers and prison guards among its members.

That must make for an interesting annual meeting, Mr. Chaison said with a laugh.

Barbara Madeloni, president of the Massachusetts Teachers Association, said she was pleased that AMSA educators chose to join a union. Union membership, she said, will improve charter school teachers' working conditions.

"Charter school teachers are really suffering tremendously," she said. "They are leaving or being forced out after a year or two. And the ones that stay, they get burnt out."

Ms. Madeloni said the MTA passed a resolution at its annual meeting to organize charter schools.

So far, though, unions have made little headway among the state's charter school teachers.

A 12-member union at the Conservatory Lab Charter School Union in Brighton, founded in 2010, was recently decertified.

Another charter school union, at the Cape Cod Lighthouse Charter School in Harwich, has never negotiated a contract and has been described as dormant.

Maybe the staff at AMSA are on the cutting edge of a revolution for the state's 71 charter schools — unionization. Or maybe not.

Aaron Nicodemus can be reached at aaron.nicodemus@telegram.com. Follow him on Twitter at @anic89.