A Massachusetts high school teacher who was chosen National Teacher of the Year on Thursday is believed to be the first from a charter school to win the award.

Sydney Chaffee, who teaches at the Codman Academy Charter Public School in Boston, was selected for the honor by the Council of Chief State School Officers. Last year, she became the first charter school teacher to be named Massachusetts Teacher of the Year, and the council believes she is the first national award winner.

Chaffee, 34, teaches ninth-grade humanities at Codman Academy, which serves inner-city students in Boston's Dorchester neighborhood. Her class teaches history and language arts by examining moments of injustice in the past, she said, ranging from decades of racial segregation in South Africa to the Haitian Revolution.

"For me, education is a tool for social justice," Chaffee said. "We arm students to fight for the world that they want, and we arm them to fight for justice."

She also leads the school's partnership with the Huntington Theatre Company in Boston, which provides acting classes to all freshmen and sophomores at the school. It's meant to help students find their voices and learn communication skills. It's also a soft spot for Chaffee, who fondly remembers acting in her high-school plays and later performing as a stand-up comic and in improv comedy.

"Some of that training has helped me as a teacher because, being a teacher, you're performing all the time in front of your kids, and figuring out how to hook them and how to get them engaged," said Chaffee, who now spends most of her spare time raising her 2-year-old daughter with her husband.

At an urban school where many students come from low-income families, it's Chaffee's job to prepare incoming students for high school academics. Year after year since she was hired in 2007, Chaffee has succeeded, said Brendan O'Connell, principal of the upper school at Codman Academy.

"Over the course of her career, she's been so good at pushing students who are challenged to see new things in themselves, to overcome, to do things that they thought they never could," O'Connell said. "She has this intense belief that the students can do it."

Chaffee's colleagues say her humility is part of her charm. Asked what sets her apart from other teachers, she questioned whether she's all that different.

"I think that one of the reasons I was selected was not because I stand out, but because I do a lot of the things a lot of great teachers do," she said. "I work really hard to build relationships with students. That's how we can build a foundation on which we can then push them to take risks and grow."

The winner of the national award is traditionally recognized by the president at a White House ceremony soon after being chosen. The Council of Chief State School Officers says no White House event has been confirmed, yet.

Chaffee was one of four finalists for the national award. The others were in California, Maryland and Wisconsin.